O.C.  BERKELEY 
ENGINEERING  LIBRARY 


; 

E 


Donald  Ifcnilton  ilo&iughlin 


VOL.    I 


A  thoBis  i>re8«nted  in  partial  fulf  lllaont  of  th« 
r«tfair«aents  f c  -  the  decree  of  3)ootor  of  Philoeophy. 

Hi"rv  rd  University. 
1917. 


i. 


CONTENTS 


Part  I.  General  Introduction. 
Importance  of  the  subject. 
Statement  of  the  problems. 
Treatment  of  the  subject. 
Field  and  laboratory  work,  and  acknowledgments. 

Part  II.  Mineralogy  of  Bornite. 
History. 
Physical  properties. 

Crystal  forms. 

Other  properties. 
Cherrical  properties. 
Composition  of  bornite. 


) 


P....-S 

1-14. 

1-3. 

3-6. 

7-10. 
10-14 . 

15-40. 
15-36. 
27-39. 
27-28. 
28-29. 
30-31. 
33-40. 


Part  III.   Descriptions  of  deposits. 
Magiratio-pneun.atolytio  deposits. 

Ookiep,  Little  Namaqualand,  South  Africa. 
La  Fleur  Mt.,  Danville  District,  Washington, 
Introduction 
Geological  relations. 
Mineralization. 

Ores  in  the  syenite  dikes. 
Mineralization  in  the  vail  rocks. 
Ore-minerals. 


41-98. 

41-143, 

41-44 

45-57 

45-46 

46-48 

43-52 

43-49 

49-50 

50-52 


il 


Contents  —  Continued. 

Pages 

Oxidation  and  Enrioiuceut.  52-53. 

Discussion.  53-57. 

Diagram  of  mineral  sequence.  55. 

Summary.  56-57. 

Evergreen  Mine,  Gilpin  Co.,  Colorado.  58-87. 

Introduction.  58-63. 

Situation.  5e- 

Development .  58-59 . 

Literature.  59-60. 

Physiographic  feature!.  61-63. 

General  geologic  features.  62-63. 

Description  of  the  deposit.  63-76. 

Associated  rooks.  63-67. 

Prirrary  mineralization  and  rook  alteration.    67-73. 

Secondary  products.  73-76. 

Summary  and  Discussion.  76-86. 

Mineral  list  and  diagram  of  sequence. arid  77-82. 
discussion. 

The  origin  of  the  deposit.  82-85. 

The  origin  of  the  ohaloooite.  35-86. 

Conclusions.  86-87. 

Copper  Kt.,  Similkameen  District,  British  Columbia. 88"9 8 • 

Introduction.  8"91* 

Situation.  88' 


iii. 


Contents  —  Continued. 

Pages 

Summary  of  previous  work.  88-91. 

Rooks  associated  with  the  ores.  91-93. 

Ores  and  rock-alteration.  93-95. 

Oxi lation  and  enrichment.  95-97. 

Sumrcary.  97-98. 

En^ele,  Plumas  Co.,  California.  99-143. 

Introduction  and  conclusions.  99-103. 

General  descriptive  treatment  —  102-.125. 
country  rock,  ores,  and  alteration. 

Magmatio  period.  103-104. 

Solidification  of  norite.  102-104. 

Pneutnatolytio  period.  104-110. 

Alteration  of  norite.  104-105. 

Formation  of  segregations.  105-106. 

Formation  of  pegmatites.  105-107. 

Dynamic  changes.  107-108. 

Development  of  iron  oxides.  108-109. 

Development  of  pneutoatolytic  sulphides.  109-110. 

Intense  hydrothermal  period.  110-116. 

Development  of  chlorite,  serioite,  andepidote J.10-113. 

Development  of  hydro thermal  sulphides.  113-116. 

Late  hydrothermal  period.  116-118. 

Development  of  zeolites  and  carbonates.  116-118. 

Period  of  oxidation  and  enrichment.  118-123. 


iv. 


Contents  —  Continued^ 

Pages 

Zone  of  coaplete  oxidation.  119. 

Zone  of  sulphide  enrichment  .  119-123, 

Discussion.  133-141. 

Diagram  of  mineral  sequence.  133. 

Genet  io  classification  of  the  deposit.  133-136. 

Origin  of  the  ohalcooite.  126' 

Chalcooite  clearly  of  replacement  origin.  1  £6-134. 

Chaloocite  in  the  graphic  structure.  134-135. 

The  Superior  Mine.  135-141. 

Summary.  141-143. 

Contaot-rretarcorphio  deposits.  144- 


Seven  Devils,  Idaho.  ^~ 

White  Horee,  Yukon  Territory.  149-153. 
Marble  Bay  Mine,  Texada  Island,  British  Columbia.  154-170. 

Introduction.  154-157. 

1  KA_ 

Situation. 

Development.  154-155. 
Literature. 

General  geology.  156-157. 

Rocka  and  ores  at   the  Marble  Bay  Mine.  157-164. 

Linestone.  157 

Porphyry.  158-161  : 

Primary  ore  -minerals.  161-163, 

Secondary  sulphides.  183-164. 


Contents  —  •  Continue.!. 


Distribution  ~ni  origin  of  the  ore-bodies. 
Mineral  sequence. 
Origin  of  secondary  sulphides. 
Summary. 
Biabee,    Arizona. 

Primary  features  of  the  ores. 
Secondary  alteration. 
Summary. 

Deposits  of  hydr  ©thermal  origin. 
Magma  Mine,  Superior,  Arizona. 
Introduction. 
Situation. 
Mines. 
Literature. 
Acknowledgments  . 
General  geology. 

Sadic.entary  column. 
Igneous  rooks. 
Structure. 

Primary  mineralization. 
Fora  of  ore  -body. 
Distribution  of  ore  -minerals. 
Mineral  sequence. 


Pages 

164-169.  <. 
164-165. 
165-168. 
168-169. 
169-170. 
171-173. 
173- 
173-177. 
177-178. 

179-311. 

179-307. 

179-181. ' 

179-180. 

180-181. 

181- 

181. 

182-187. 

182. 

183-185. 

185-187. 

187rl93 . 

187- 

187-189, 

189-193. 


vi. 


Contents  —  Continued. 

Oxidation  and  enrichment. 
Water  level. 
Oxidized  ores. 
Secondary  sulphides. 
Discussion  ar.d  suMr.ury. 

Diagrarc  of  mineral   sequence. 
Origin  of  the  priirary  ores. 
Origin  of  the   secondary  ores. 
Conclusions. 

The  Kenneoott  district,   Alaska. 
Introduction. 
Situation. 
Vines. 
Literature. 
Aoknowle  dgments . 
General  geology. 
Nikolai  greenstone. 
Chitistone  limestone. 
McCarthy  shale. 
Post-Triuasio  deforn.ation. 
Keanecott  formation. 
Porphyries. 
Wrangell  voloanios. 
Structures. 


.-.-... -3  3 
193-197. 
193- 
193-195. 
195-197. 
198-207. 
158- 
199-203. 
303-207 . 
207  T 
208-311 . 
208-213. 
208-209. 
209-210. 
211-213. 
213- 
213-218. 
213-216. 
216-218. 
318- 
218- 
218-220. 
220-223. 
223-224. 
224-226. 


. 


vli. 


Contents   —  Continued. 

Pages 

Physiography.  237-334. 

Pro-Chitistone  surface.  337-338. 

Pre-Kenneoott  surface.  338- 

Pre-Tertiary  voloanio  surface .  338-230. 

Pro-Glacial  surface.  330-331. 

Glaoiatlon  and  the  present  surface.  331-234. 

Mineralization  in  the  greenstone.  234-343. 

Form  of  deposits.  234-335. 

Gangue  minerals.  235-236. 

Rook  alteration.  236- 

Primary  ore -minerals.  236- 

Seoondary  alteration.  237-338. 

Deposits  near  the  Bonanza  Mine.  238-239. 

Kuskulana  District.  239-241. 

Kotsina  District.  241-242. 

Suiroary  of  important  features.  343- 

Ore-deposits  in  the  Chitistone  limestone.  243-379. 

Geologic  situation  of  the  mines.  243- 

Struotural  relations.  244-347. 

Gangue  minerals  and  rook  alteration.  247-248. 

Ore -mineral s.  248-266. 

Relative  abundance.  248-349. 

Chaloooite.  249-255. 


viii, 


Con  tent  8  "r  Continued. 


Covellite.  255-357. 

Bornite.  257-263. 

Enargite.  263-364. 

Chaloopyrite.  264~ 

Luzon!  te.  264-265. 

Tennantite.  265-266. 

Pyrite.  366- 

Sphalerite  and  galena. 

Conoentrio  and  banded  etruoturee.  367-268. 
Oxidation. 

Dietribution.  269-370 

Underground  temperatures. 

Water  level.  371~ 

Minerals  due  to  oxidation.  271-279. 

The  origin  of  the  oopper  deposits  in  the  279-265. 
greenstone  . 

Similarity  to  other  regions.  279- 

Source  of  the  netala.  279-380. 

Theories  of  concentration  of  oopper  in  280-285. 

basic  lavas. 

Resume  .  285- 

The  origin  of  the  oopper  -deposit  a  in  the  286-294. 
limestone. 

Summary  of  significant  features.  286-288. 

Origin  of  the  primary  ore.  289-294. 


is. 


Contents  — -  Continual. 


The  nature  of  the  ohaloooite.  294-307. 

Evidence  fron-  the  studies  of  the  294-296. 

Geophysical  Laboratory. 

Evidence  that  the  ohaloooite  is  a  297-299. 

replacement  of  bornite. 

Secondary  versus  primary  origin  9-307. 

for  the  ohaloocite. 

Table  of  arguments  concerning  the 
origin  of  the   ohaloooite. 

Summary  arid  conclusions.  07-311. 


Part   IV.      General  Discussion.  312-401. 

General  features  of  bornite  deposition.  312-334. 

Bornite  in  deposits  of  magnetic  or  s- 
pneumatolytic  origin. 

Some  properties  of  inagtratio  sulphide  ores.  312-313. 

Pyrrhotite-chaloopyrite -bornite  ores.  -316. 

Pneumatolytio  ores.  316-319. 

Bornite   in  deposits  of  oontaot-retansorphio  319-322. 
origin. 

Bornite  in  deposits  of  hydrotherical  origin.  322-325. 

Bornite   in  replacement  ores  in  unaltered 
litres  tone. 

Bornite  deposited  from  cold  meteoric  solutions.  '330< 

Ores  of  the   "Red  Beds"  type.  326-328. 

Secondary  bornite.  328-330. 

Sutr,niary  of  conditions  under  which  bornite  '-334. 
has  bean  forired. 


X. 


Contents  —  -  Continued. 

Pages 

The  alteration  of  bornite.  335-353. 

Field  relations.  335-339. 

Selective  enrichment.  339-343. 

e 

Uicroaoopio  relations.  343-355. 

Sinrple   structures.  344-345. 

The  lattice  structure.  345-353. 

Residual  structures  related  to  the  353-355. 
lattice   structure. 

Chaloooite   derived  from  bornite.  355-358. 

Tiie  graphic  structure.  360-401. 

Description.  360-361. 


minerals  in  the  graphic  structure  351-363. 
with  bornite. 

Chaloooite  in  the  graphic  structure.  363-398. 

Li  terature  .  364-366  . 

Summary  of  evidence.  367-368. 

Argument  favoring  a  contemporaneous  origin.  369-375. 

Arguments  favoring  a  replacement  origin.  375-382. 

The  hypothesis  of  contemporaneous  origin  283-388. 
modified  by  replacement. 

The  hypothesis  of  selective  replacement.  383-398. 

Summary  and  oonelusions.  398-401. 


4  S  T I 


X  1  £  1  L         I  fl  I  ii  0  D  U  0   3?  I  O.fl 


GENERAL  INTRODUCTION 
Importance  of  the  Subject 

In  dealing  with  the  numerous  problems  connected  with 
a  comprehensive  geologic  study  of  the  sulphide  ores  of  copper, 
it  has  been  found  that  some  of  the  most  intricate  and  puzzling 
questions  encountered  are  those  associated  with  the  mineral 
bornite.   In  the  case  of  the  other  ore-minerals,  the  relations 
observed  either  in  the  field  or  under  the  microscope  are  at'  a 
rule  relatively  simple  and  constant,  and  generally  yield  a 
definite  but  limited  amount  of  information.  Bornite,  however, 
exhibits  under  diverse  conditions  a  variety  of  features  and 
a  complexity  of  relationships  which  are  difficult  of  inter- 
pretation, but  which  are  of  great  significance  when  understood. 
The  questions  which  arise  concerning  this  mineral  range  from 
broad  problems  demanding  knowledge  of  the  field  relations  to 
intimate  queries  concerning  the  details  of  intricate  micro- 
scopic structures,  and  an  appreciation  of  the  critical  evidence 
which  is  offered  by  associations  and  properties  of  bornite  is 
often  necessary  before  a  satisfactory  understanding  of  the  na- 
ture of  certain  ore-bo  lies  can  be  obtained.  A  clear  concep- 
tion of  the  genesis  of  the  ore  can  often  be  obtained  from  the 
relations  of  the  bornite  to  other  primary  sulphides  and  to  the 
gangue  minsrals,  and  the  effects  of  superficial  agencies  are 
invariably  recorded  in  the  bornite  with  far  greater  delicacy 


. 


CX?  • 

" 


.•3 


and  completeness  than  in  the  other  sulphides.  Apart  from  the 
scientific  value  of  detailed  knowledge  of  the  position  occupied 
by  bornite  in  the  processes  of  primary  mineralization,  the 
richness  of  bornite  as  an  ore  of  copper  makes  any  information 
especially  welcome  which  would  lead  to  a  more  accurate  com- 
prehension of  the  habits  and  distribution  of  the  mineral  and 
which  would  direct  prospecting  and  development  work  most  in- 
telligently. The  study  of  secondary  ores  derived  from  bor- 
nite presents  another  field  which  offers  many  open  problems 
of  both  scientific  and  practical  worth.  The  economic  value 
of  a  correct  judgment  concerning  the  secondary  or  primary 
nature  of  a  chcxloocite  ore  makes  it  of  great  importance  to 
seek  all  means  by  which  the  necessary  information  oan  be 
gained.  In  most  deposits,  ohaloocite  ores  are  believed  to 
be  of  secondary  nature,  and  all  plans  for  their  development 
are  based  upon  the  expectation  that  they  will  pass  into  lean- 
er primary  material  at  relatively  shallow  depths.   In  the 
deposits  in  which  the  nature  of  the  chalcocite  is  in  question, 
the  mineral  Has  beeii  found  almost  invariably  to  be  in  close 
association  with  bornite,  and  the  problems  of  its  origin  have 
resolved  themselves  largely  into  studies  of  its  relations 
to  this  mineral. 


1.  Throughout  this  paper  the  terms  primary  and  secondary 
are  used  only  in  their  geologic  sense.  The  former  is  re- 
served for  processes  ani  products  associated  with  the  orig- 
inal mineralization;  the  lutter  for  processes  and  products 
dependent  on  superficial  alteration. 


- 

. 


. 
, 

' 


It  has  therefore  been  considered  desirable  to  as- 
certain by  all  available  means  the  facts  concerning  the  oc- 
currence and  relations  of  bornite,  and  to  gain  an  understand- 
ing of  their  significance.   To  do  this  hae  been  the  object 
of  this  investigation. 

Although  many  of  the  broad  features  of  bornite 
ores  have  been  recognized  for  rrany  years,  the  reoent  applica- 
tion of  the  metallographic  microscope  to  the  study  of  opaque 
minerals  has  revealed  so  many  unsuspected  relations  that 
very  little  of  the  older  literature  concerning  the  paragen- 
esis  of  bornite  can  be  accepted  without  reservation.  Conse- 
quently, with  the  exception  of  the  work  of  a  few  recant  in- 
vestigators, the  study  of  the  relations  of  bornite  by  modern 
microscopical  methods  opens  an  almost  new  field  in  which 
thorough  exploration  is  fully  Justified  by  the  promise  of 
results  of  both  scientific  and  economic  value. 

Statement  of  the  Problems 

From  both  a  scientific  and  an  economic  standpoint, 
the  chief  problems  associated  with  the  deposits  studied  may 
be  divided  into  two  groups: 

(1)  those  concerned  with  the  nature  of  the  primary 
mineralization  arid 

(2)  those  concerned  with  the  character  of  the  second- 
ary alteration. 

In  the  first  group,  the  establishment  of  the  range  of  condi- 
tions under  which  bornite  may  form  is  the  most  general  prob- 


, 

. 
. 

; 
- 

30. 
. 
' 

. 

•    • 

. 


lem,  and  all  others  are  units  leading  to  its  solution.   In 
tbe  oase  of  each  deposit,  knowledge  must  be  obtained  con- 
cerning the  form  of  the  ore-bodies,  their  relation  to  the 
neighboring  rocks  and  structural  features,  and  the  position 
of  bornite  in  the  sequence  of  gangue  and  ore -minerals.  From 
the  interpretation  of  the  evidence  afforded  by  these  points 
and  others,  the  genesis  of  the  deposit  may  be  inferred  in 
most  cases  with  reasonable  certainty. 

The  second  group  of  problems,  viz.  those  related 
to  the  secondary  changes  due  to  the  superficial  alteration 
of  an  ore -body  brings  up  a  greater  number  of  queries.  The 
determination  of  the  importance  of  bornite  as  a  product  of 
surface  processes  is  one  of  the  most  definite  problems  which 
has  been  encountered.  The  suspicion  in  many  mining  camps 
that  the  bornite  ores  are  of  secondary  origin  emphasizes  the 
value  of  positive  information  on  this  point  and  it  is  be- 
lieved that  evidence  of  a  final  nature  is  presented  in  these 
studies.   The  distribution  of  the  secondary  sulphides,  the 
depth  to  which  enrichment  can  extend  in  bornite  ores,  com- 
parisons of  similar  influences  on  pyritio  and  other  ores, 
and  the  influence  on  the  alteration  of  bornite  of  numerous 
factors  such  as  climate,  physiography,  wall  rock,  rook 
alteration,  and  structure  all  offer  problems  depending 
largely  upon  field  observations  for  solution.  On  the  other 
hand,  the  degree  of  replacement  by  secondary  sulphides  which 
the  bornite  has  suffered,  the  relative  ease  of  bornite  en- 


5. 


richment  compared  with  the  other  sulphides,  and  the  rela- 
tions between  the  various  ore  and  gangue -minerals  are 
questions  which  are  roost  definitely  settled  in  the  labora- 
tory. The  microscopic  relations  of  chaloocite  to  bornite 
offer  a  great  number  of  important  problems,  among  which 
the  most  important  are  the  interpretation  of  the  "lattice 
structure",  in  which  the  chaloooite  penetrates  the  bornite 
as  a  regular  grill  of  intersecting  lines  or  the  "graphic 
structure",  in  which  the  two  minerals  are  associated  in 
eutectic  like  patterns  on  the  polished  surface.   In  seeking 

the  meaning  of  these  difficult  relations,  numerous  minor 

/i 
£/ 

features,  such  as  residual  "spines"  of  bornite  in  chaloooite, 

• 

rims  of  bornite  about  partially  altered  grains,  ""haloa". 
and  "hazy  boundaries"  all  demand  their  share  of  attention, 
and  add  the  weight  of  their  evidence  to  the  general  attack 
on  the  problems  of  bornite  alteration. 

The  important  question  of  primary  chaloocite  ia 
closely  bound  up  with  the  interpretation  of  the  various 
relations  between  chaloocite  and  bornite.  The  deep  enrich- 
ment in  bornite  ores,  the  inheritance  of  bornite  structure 
by  ohalcooite,  the  two  etoh  patterns  in  chalcooite,  and  the 
origin  of  the  lattice  and  of  the  graphic  structures  are 
points  which  must  be  understood  before  a  final  verdict  can 
be  given.   The  economic  importance  of  the  problem  ie  ob- 
vious. Experience  in  many  camps  has  shown  that  the  rich 
chalcocite  ores  are  usually  of  secondary  origin,  i.e. 


• 

. 

' 


• 
- 
• 


, 


6. 


formed  by  the  action  of  meteoric  waters  descending  through 
the  deposit  from  the  surface.  Consequently  they  are  ex- 
pected to  give  way  at  relatively  shallow  depths  to  leaner 
primary  sulphides,  which  may  possibly  be  too  low  in  copper 
to  be  worked  profitably.   If,  however,  it  can  be  established 
from  a  study  of  the  relations  between  the  sulphides  on  the 
upper  levels,  that  the  ohaloooite  is  a  primary  constituent 
of  the  ore,  its  continuance  with  depth  would  be  expected, 
and  all  plans  for  the  development  of  the  deposit  would  be 
influenced  by  this  knowledge.  If  the  chaloocite  is  second- 
ary, it  becomes  of  economic  importance  to  be  able  to  pre- 
dict the  nature  of  the  change  where  the  superficial  sul- 
phides yield  to  the  primary  ores.   If  it  can  be  determined 
that  the  chaloocite  is  largely  a  replacement  of  bornite, 
the  decrease  in  copper  per  centage  of  the  ore  will  be 
relatively  small,  and  will  ordinarily  permit  profitable 
operations  to  be  carried  into  the  primary  ore;  if,  however, 
the  ohaloooite  is  a  replacement  in  part  at  least,  of  pyritic 
material,  the  decline  in  value  is  much  greater  and  may  not 
allow  the  profitable  mining  of  the  deeper  ore.  Although 
evidence  on  these  points  may  be  gained  by  drilling  or  other 
exploratory  methods,  it  is  believed  that  in  many  cases  reli- 
able conclusions  may  be  gained  at  less  expense  from  micro- 
scopical studies  of  samples  from  the  upper  parts  of  the  de- 
posits. 


Treatment  of  the  Subject 

The  aoouraoy  of  any  conclusions  of  general  nature 
whioh  may  be  drawn,  depanda  directly  upon  the  degree  to 

V 

which  tiia  material  selected  as  a  fair  sample  of  all  occur- 
rences, and  consequently  the  attempt  has  been  made  to  in- 
clude among  the  deposits  studied,  examples  from  all  types 
of  ores  in  -thioh  bornite  is  an  important7  mineral.   The  de- 
posits chosen  for  detailed  investigation  include  several 
in  whioh  the  bornite  is  believed  to  be  of  either  magmatio 
or  pneumatolytic  origin,  others  considered  to  be  typical 
examples  of  oontact-rcetarrorphic  ores,  and  certain  deposits 
regarded  as  products  of  various  phases  of  hydrothermal 
action.   In  addition  the  camps  selected  are  situated  in 
widely  separated  regions,  consequently  their  ores  have 
been  subjected  to  sufficiently  diverse  climatic  and  physio- 
graphic influences  to  offer  as  great  a  range  in  conditions 
affecting  the  development  of  secondary  sulphides  as  is 
necessary  to  insure  the  general  application  of  the  rela- 
tions observed. 

Before  the  main  part  of  the  paper  is  presented, 
a  short  discussion  is  given  cf  some  features  of  general 
mineralogical  interest  in  connection  with  bornite.  A 
brief  historical  account  of  the  mineral  is  presented  with 
a  summary  of  older  analytic  and  synthetic  work,  whioh  al- 
though of  little  present  value,  is  of  some  interest  as  a 
record  of  the  difficulties  caused  by  unsuspected  impurities 


8, 


in  the  materials  available  for  study* 

The  descriptions  of  a  series  of  ore-deposits  in 
which  bornite  plays  an  important  part,  either  as  an  ore-min- 
eral or  as  a  sigriigioant  factor  in  their  history,  form  an 
important  part  of  paper.  Where  definite  problems  can  be 
handled  in  the  case  of  individual  deposits,  they  will  be 
considered  there;  if  more  general  evidence  is  required  for 
their  solution,  they  will  be  reserved  for  the  later  chapters 
in  which  knowledge  from  the  whole  field  may  be  brought  to 
bear  upon  them*  The  different  deposits  are  treated  with 
various  degrees  of  thoroughness.  In  oases  in  which  the 
field  work  and  collections  offered  new  and  more  definite 
information  than  had  been  previously  published,  the  details 
are  given  at  greater  length  than  for  those  deposits  in 
which  the  investigation  is  limited  to  a  study  of  older 
collections,  or  merely  to  the  literature.  A  few  especially 
interesting  deposits,  such  as  those  at  Engels,  California 
and  at  Kennecott,  Alaska,  have  been  described  more  fully 
than  many  of  the  others,  but  their  unusual  features  are  be- 
lieved to  justify  the  prominence  given  them.  Other  impor- 
tant deposits,  however,  such  as  t;  oae  at  Butte,  Montana,  and 
Bisbee,  Arizona,  have  been  dealt  with  very  briefly,  even 
though  their  ores  present  many  of  the  most  intricate  problems 
associated  with  bornite.   The  chief  features  of  the  great 
ore -bodies,  however,  have  already  been  given  a  prominent 
place  in  the  literature,  and  the  time  available  was  too 


short  to  make  a  study  of  the  field  conditions  which  would  add 
any  additional  knowledge  of  value.  The  handicap  of  being 
forced  by  lack  of  time  to  slight  these  important  deposits  is 
greatly  offset,  however,  by  reliable  information  that  is 
available  from  recent  work  by  other  investigations.  A  de- 
tailed paper  on  the  Bisbee  ores,  based  on  an  intimate  knowl- 
edge of  both  field  and  microscopical  relations  supplies 
valuable  data  which  may  be  accepted  without  question,  and 
intensive  studies  of  Butte  material  which  are  being  carried 
on  in  this  laboratory  constantly  add  trustworthy  evidence  on 
the  problems  presented  by  those  deposits. 

The  order  in  which  the  camps  are  described  is 
based  in  a  broad  way  upon  the  interpretation  of  their  origin. 
The  ores  which  are  believed  to  be  rrost  closely  related  to 
magmatio  conditions  are  discussed  first;  they  are  followed 
by  the  contaot-metamorphio  deposits,  and  later  by  descrip- 
tions of  ores  formed  under  various  stages  of  hydrothermal 
action.   The  descriptions  are  thus  arranged  according  to 
decreasing  intensity  of  primary  conditions  of  origin,  but 
on  account  of  the  complex  and  transitional  nature  of  some 
of  the  deposits,  their  grouping  must  be  somewhat  arbitrary. 
For  example,  the  Engels  ore -body  furnishes  evidence  of 
sulphides  formed  under  varying  conditions  from  pneumatolytio 
to  late  hydrothermal,  but  on  account  of  the  greater  interest 
attached  to  the  earlier  products,  it  is  presented  with  the 
pneumatolytic  deposits,  although  the  larger  part  of  the 


10, 


commercial  ere  was  foriwd  under  hydrothermal  conditions. 
In  the  fourth  section  of  the  paper,  the  impor- 
tant primary  relations  of  the  bornite  ores  are  discussed, 
and  the  common  properties  and  general  range  of  genetic 
conditions  favorable  for  the  production  of  bornite  are 
emphasized.  A  second  divison  of  this  section  is  concerned 
with  the  problems  associated  with  this  alteration  of  bornite, 
such  as  the  relative  ease  of  replacement  by  ohaloooite,  the 
field  distribution  of  the  chalcocite,  and  the  significance 
of  the  different  types  of  replacement  structures.  The  final 
divison  of  the  section  is  devoted  to  the  much  debated  ques- 
tion of  the  primary  or  secondary  origin  of  the  chalcocite 
in  the  graphic  structure  with  bornite.   The  evidence  and 
the  various  arguments  are  summarized,  and  two  explanations 
are  proposed.  The  paper  closes  with  a  summary  of  the  results 
and  conclusions  in  individual  cases  and  in  general. 

Fisld  and  Laboratory  Work,  and  Acknowledge  merits 

The  work  of  this  investigation  of  bornite  ores  has 
extended  over  a  period  of  about  two  years,  but  the  time  has 
been  somewhat  diminished  by  other  smaller  problems  and 
academic  duties.   During  two  summers  in  the  field, attention 

has  been  largely  directed  toward  the  occurrences  of  bornite. 
A  large  fraction  of  the  other  months  have  been  consumed  in 
laboratory  studies  of  the  material  and  information  gathered. 
The  work  embodied  in  this  paper  is  part  of  an  ex- 


W    c 


11, 


tensive  study  of  the  sulphide  ores  of  copper  which  is  being 
carried  on  by  the  Secondary  Enrichment  Investigation,  and 
consequently  a  wider  range  of  information  wa8  available 
upon  which  arguments  and  conclusions  could  be  based  than 
the  special  field  emphasized  in  direct  connection  with  the 
problems  of  bornite. 

The  deposits  near  Kenneoott,  Alaska,  were  visited 
in  the  summer  of  1915,  in  the  company  of  Messrs.  L.  C. 
Graton  and  A.  M.  Bate man.   The  larger  part  of  the  time  was 
spent  in  studying  the  great  ohalcocite  ore -bodies  of  the 
region  in  which  bornite  is  a  small  but  very  significant 
constituent,  and  a  short  time  was  available  for  a  study  of 
the  smaller  ohaloopyrite -bornite  deposits  in  the  greenstone 
of  the  neighborhood.  In  addition  to  material  collected  in 
1915,  specimens  and  information  from  parts  of  the  region  I 
was  unable  to  visit  have  been  placed  at  my  disposal  by  Mr. 
Alfred  Wandtke  to  whom  I  owe  my  sincere  thanks. 

A  broader  range  of  country  was  covered  in  the 
summer  of  1916,  and  a  valuable  collection  of  material  was 
obtained  from  most  of  the  important  camps  in  western  United 
States  in  which  bornite  is^oteworthy  ore-mineral.  The  list 

n 

of  mines  and  districts  in  which  the  occurrence  of  bornite 
was  especially  studied  in  1916  includes:   Engels,  Plumas 
Co.,  California;  Marble  Bay  Mine,  Texada  Island,  B.  C.,  the 
Similkameen  district,  British  Columbia;  the  Boundary  District, 
British  Columbia  and  Washington;  Butte,  Montana;  the  Ever- 


^  . 


12, 


green  Mine,  Gilpin  Co.,  Colorado,  and  Biebee,  Superior,  and 
Morenci,  Arizona.  Visits  were  also  wade  to  the  following 
camps  in  which  bornite  is  of  minor  importance:   Rossland, 
B.  C.,  Bingham  arid  Tintic,  Utah;  Ely,  Nevada)  Tularoaa, 
Tyrone  and  Santa  Rita,  New  Mexico;  and  Jerome,  Ray,  Miami, 
Globe,  and  Morenci,  Arizona.  A  considerable  part  of  the 
expenses  of  the  work  were  covered  by  a  Sheldon  Traveling 
Fellowship  from  Harvard  University.   In  several  of  the 
most  important  camps,  the  field  observations  were  made  in 
the  company  of  Messrs.  L.  C.  Graton  and  A.  Locke,  and  from 
their  intimate  knowledge  from  the  results  of  previous  work 
of  the  Investigation,  I  gained  a  far  clearer  conception  of 
the  field  conditions  than  coull  have  possibly  been  obtained 
in  any  other  way.  In  all  places  the  hospitality,  character- 
istic of  the  mining  profession, expressed  itself  in  the  kind 
efforts  that  were  made  to  give  me  all  opportunities  to  study 
the  mine  workings,  and  to  acquaint  me  with  the  local  features 
of  geologic  importance.  Especial  thanks  are  due  to  Messrs. 
Stephen  Birch,  W.  H.  Seagrave  and  H.  D.  Smith  of  the  Kenne- 
cott  Copper  Corporation  for  many  courtesies  extended  dur- 
ing our  work  in  Alaska  in  1915;  to  Mr.  John  Reinmiller  at 
the  Engela  Mine,  Calfornia;  to  Mr.  A.  F.  Eastman  of  the 
Taooma  Steel  Co.,  to  whom  I  owe  the  privilege  of  visiting 
the  Marble  Bay  Mine  on  Texada  Island;  to  Messrs.  Patrick 
Crane  and  H.  E.  Doelle  of  the  British  Columbia  Copper  Com- 
pany, whose  guest  I  was  for  several  days  on  Copper  Moun- 


13, 


near  Prinoeton,  3.  C.,  arid  at  the  Mother  Lode  Mine;  to 
Messrs.  C.  F.  Martin  and  Nelson  at  the  Granby  Companies' 
properties  at  Phoenix,  B.  C.;  to  Mr.  0.  S.  Trombley,  whose 
cabin  I  shared  on  the  slopes  of  La  Fleur  Mt.  in  the  Boundary 
District,  a  few  mi  lea  from  the  line  in  Washington;  to  Mosars. 
Arthur  Linforth,  D.  A.  Hall  and  other  members  of  the  geolog-- 
ical  staffs  at  Butts,  Montana;  to  Professor  H.  B.  Patton  of 
the  Colorado  School  of  Mines  at  Golden,  with  whom  I  apsnt 
two  enjoyable  and  instructive  days  on  a  trip  to  the  Ever- 
green Mine  in  Gilpin  Co.,  Colorado;  to  Messrs.  Y.  S.  Bonillas 
and  Leon  Fenohere  of  the  Copper  Queen  Mining  Co.,  at  Biabee 
arid  Mr.  Ira  B.  Joralemon  of  the  Calumet  and  Arizona  Mining 
Co.,  at  Warren,  Arizona,  who  kindly  showed  me  the  aoat  inter- 
esting parts  of  their  ore-bodies  in  connection  with  my 
problems;  and  to  Messrs.  W.  C.  Browning  and  I.  A.  Ettlinger 
of  the  Magma  Copper  Company  at  Superior,  Arizona.  To  many 
others  at  the  various  camps  visited,  I  am  indebted  for 

many  courtesies,  for  which  I  wish  to  express  cy  thanks* 

I  am  indebted  to  Mr.  Joseph  Murdoch  for  many  excel- 
lent photographs  of  bornite  structures,  which  were  made  by 
him  during  his  work  on  the  staff  of  the  Secondary  Enrichment 
Investigation,  and  also  to  Mr.  W.  L.  Whitehead  for  several 
photographs  of  material  from  the  Engels  Mine  . 

My  thanks  are  especially  due  to  Professor  L.  C. 
Graton  for  the  thoughtful  and  valuable  criticism  that  he  has 
given  all  parts  of  this  work,  both  in  the  field  and  in  the 


14. 


laboratory,  and  in  connection  with  the  preparation  of  the 
manuscript,  for  which  I  ans  very  grateful. 


P  A  It  2 


MIBBEALOGY       0?       B  0  R  M   I  g  JS 


15, 


PAST  ii.  :.:i:i:HALoaY  o? 

History 

The  existence  of  "oopper  pyrites"  has  probably  "been 
known  since  the  metal  was  first  won  from  sulphide  ores,  but  the 
similarity  in  color  between  bornite  and  ohalcopyrite  when  they 
are  tarnished  made  their  separation  into  distinct  mineral 
species  uncertain  almost  until  modern  times.  There  is  evidence, 
however,  that  the  Identity  of  bornite  was  suspected  as  early  as 
1546,  for  Georgius  Agrioola  in  his  famous  TO rk  ££.  Natura  Fossilium 
printed  in  that  year,  mentioned  two  minerals,  "Pyrites  aurel 
colore"  and  "Pyrites  aerosus" ,  which  may  be  correlated  fairly 
certainly  with  ch&lcopyrite  and  bornite.  The  distinction  of  the 
tarnished  variety  of  "oopper  pyrites'1  by  a  separate  name  is 
significant,  and  warrants  the  addition  of  bornite  to  the  long 
list  of  minerals  first  recognized  by  Agrioola.^ 

Appreciation  of  the  distinct  character  of  bornite  is 
probably  found  in  Johann  Friedrioh  Henokel's  "Pyritologia'f  or 
"Kies-Historie" ,  published  in  Leipzig  in  172.5,  but  the  vagueness 
of  his  statements  and  the  variable  application  of  the  names  he 
uses,  makes  his  contribution  even  less  definite  than  the  des- 
criptive terms  employed  by  Agrioola.  The  sentences  containing 


1.  The  source  of  information  concerning  Arrioola  and  his 
v.orks  most  easily  available  is  the  excellent  translation  of 
le  Re  :.ietallloa.  (1556)  by  Herbert  Clark  Hoover  and  Lou  Henry 
Hoover  (London, 1912 ).  On  page  109,  a  summary  of  minerals  in  De 
Natura  Fossilium  is  given. 


16. 


the  probable  reference  to  bornite  are  moat  satisfactorily 
given  in  hia  own  language,  and  are  as  follows:  -  -  Xupffer- 
glass  iat  noon  dunokler [than  ?ahlerz,  previously  described] 
und  neiget  sioh  gar  zur  Sohwartze,  aus  Ursachen,  weil  as 
sehr  iisenschussig  1st,  dergleichen  mir  von  den  awfllff 
Sohluaseln  bekannt;  und  Kupf f er-  ,asur  nimmt  sioh  nebst 
seiner  dunckeln  Farbe  ait  seinen  stahlblauen  tfarben  aus, 
wiewohl  der  I.Ii-sbrauch  auoh  eingefuhret  hat,  ein  sonst 
gelb-grunlichtes  Xupffer-iSrz,  ao  nur  auf  Klufften  mit 
blauen  Farben  spielet,  Kupffer-Lasur  zu  nennen,  und  es 
scheinet  dass  Kupffergiass  bei  den  Alten  keine  andere 
Saohe  als  Kupffor-uasur  bedeutet  habe,  sondern  gleichwie 
aus  Glaaur  Lasur,  als  aus  Glasur  auoh  Glass  naoh  der  alien 
Spraohen  ge.Tieinen  von  Geschwindigkeit  in  ausspreohen 
entstehenden  Worter  Verkurtzungen,  und  2uaammenziehungen( 
entsprungon  sei.  '  She  mineral  described  by  Henokel  is 


1.   Translation;  Kupffer-glaaaCohaloooitet]  is  still 
darker  £  than  Pahlerz,  tetrahedrito,  previously  referred  to»] 
and  even  tends  toward  blackness,  because  it  has  a  high  iron 
content  due  to  causes  which  ar9  known  to  me  from  the  twelve 
keysf  ?3;  and  Kupf fer- Gasur  C  probably  bornitej  has  in  addition 
to  its  dark  color  a  steel  blue  shade,  although  the  misuse 
his  arisen  of  calling  a  usually  yellow-groen  copper-ore, 
Kupffer-lasur  which  sparkles  with  a  blue  color  only  along 
fractures  and  it  seems  the  Kupf f erglass C  chaloocite}  among 
tho  ancients  meant  nothing  less  than  Kupfer-ijusur,  just  as 
from  Glasur,   Lasur  and  from  Glasur,  Gla»8»  originated  from 
abbreviations  and  contractions  common  to  all  languages  re- 
sulting from  rapidity  in  .speech.  ._Johann  Priedrich  Henckel, 
Pyritologia,  Leipzig,  1VE5,  oage  451. 


17. 


classified  as  a  member  of  the  "^less-Holla" ,  and  Is 
therefore  a  metallic  sulphide  or  arsenide,  according  to 
his  definition.  It  is  doubtful, however, whether  the  name 
Xupffer-Lasur  was  confined  to  this  usage,  for  it  la  applied 
to  the  mineral  azurite  by  slightly  later  writers. 

The  earliest  publication  in  which  names  are  given 
which  clearly  indicate  that  the  mineral  was  commonly 
recognized  at  the  time,  is  the  Mineralogy  by  Johann 

tfottskalk  Wallerius,  printed  in  1725.  A  description  of 

1 
bornlte  appears  in  the  Latin  edition  of  1778  under  the 

heading,  Cuprum,  pyrite  fuso  mine rali sat urn,  minera  flavo 
fuaoa.  and  the  following  popular  names  are  added : - 
Swedish,  Le fever slag,  Gulbrun  kopparmalm;  French,  Mine 
de  ouivre  hepatioue;  and  German,  Braunes  kupfererz,  or 
Leberschlag.  In  the  French  translation  of  Wallerius,  by 
Paul  Thiry,  published  In  1759,  the  mineral  is  referred  to 
under  the  names:-  "Mine  de  ouivre  hepatique  ou  de  la 
oouleur  du  foye,"  vrith  the  latin  terms,  "cuprum  sulphure, 
ferro  mineral! satua,  minera  pyriticosa  fulva;  Minera 
oupri  hepatic a.  Tho  description  is  as  follows:-  "Elle 
est  d'um  jaune  tirant  sur  le  brun  on  d'une  oouleur  pale, 


1.  Page  285.  Oolore  est  flavo  fusoo,  reipsa  pyrites 
fuscus  (spec.  277}  cuprum  solutum  oontinans  parva  quantitate; 
proprletatas  pyrltae  fusci  nonlmatl  utplurimum  retinens; 
a  quibus  haeo  minera  oognosi  et  agnosoi  potost* 


18. 


et  etroitement  unie  avec  du  soufre  et  du  fer;  la 
substance  est  pyriteuse:  11  y  a  meme  lea  Haturalistea  qul 
la  mettent  au  rang  dea  pyrites;  frappee  aveo  1'acier,  elle 
ne  donne  que  peu  ou  point  d'etinoellea.   On  a  (1)  La  mine 
de  ouivre  hepatique  brune  (Minera  cupri  hepatioa  fulva) 
Slle  est  de  la  couleur  du  foye;  riche  en  cuivre,  et  d'une 
oonsistenoe  tantot  aerre'e  tantot  pen  compacte.   (2)   La  mine 
de  cuivre  hepatique  pale.  (Minera  oupri  hepatica  livida.) 
Bile  eat  preaque  blanche  et  pale  comme  de  1'etain,  d'une 
couleur  fonoee,  et  d'un  brun  tirant  aur  le  bleu;  intorieure- 

ment  elle  paro£t  composee  de  grains,  mala  extorieureraent  elle 

,  1 
aomble  feuilletee.   The  various  tarnished  colors  of  the 

bornite  are  probably  the  cause  for  the  subdivision  of  the 
species,  but  another  mineral  may  be  meant  by  the  aeoond 
description. 


1.  Translation.   It  is  of  a  yellow  color,  tending  toward 
brown,  or  a  pale  color,  and  intimately  united  with  sulphur 
and  iron;  the  material  is  pyritio:  there  are  even  some 
Naturalists  who  place  it  in  the  group  of  pyrites;  struck 
with  steel  it  gives  only  a  few  sparka  or  none  at  all.  We 
have  (1)  the  liver  brown  copper  mineral.  It  is  the  color 
of  liver,  rich  in  copper,  and  of  a  dense  to  slightly 
compact  texture.   (£)  The  pale  liver-colored  copper 
mineral.   It  is  almost  white  and  as  pale  as  tin,  of  a  warm 
color,  and  of  a  brown,  turning  toward  blue;  internally  it 
appears  granular,  but  on  the  outside  it  seems  foliated. 


19, 


Cronstedt  in  his  Mineralogy,  published  in  1758 
definitely  recognises  the  mineral,  terrain?  it  "Pyrites  oupri 
hepatious"  according  to  a  quotation  in  the  later  Latin 

edition  of  Wallerius.  In  the  Knglish  translation  by  Jin- 

1 
geet£6m  in  1770,  bornite  is  classified  in  a  subdivision 

of  the  main  group  of  copper-minerals,  headed  "With  sul- 
phurated iron,  rainera  cupri  pyritaoea,"  and  Is  described 
ae  follows:-  "Reddish  yellow,  or  liver  brown,  with  a  blue 
coat  on  tho  surface,  Ulnera  cupri  le^urea.  This  ore  yields 
between  40  and  50  %  of  copper,  and  is  commonly  said  to  be 
blue,  though  it  is  as  red  whon  freeh  broke,  es  a  rioh 
oopper  regulus." 

The  term" liver-ore"  was  probably  the  earliest  name 

in  common  use  in  England.  It  is  given  by  Forster  in  his 

2 
"Introduction  to  Liineralogy,  published  in  1768.   He 

writes,  "Liver-ore  is  a  rich  copper-ore  consisting  of  a 
good  deal  of  oopper  mixed  with  iron,  of  a  yellow  brownish 
colour,  "but  mixed  with  some  green  spots,  which  distinguish 
them  from  the  iron  ores."  The  value  of  the  mineral  as  an  ore 
of  copper  was  undoubtedly  well  jcnownat  this  time. 

1.  Axel  Predrik  Gronstedt.  An  Essay  towards  a  SyBten  of 
Mineralogy.  Translated  from  the  original  Swedish,  with  notes, 
by  oustaf  Engestrom.  London,  1770,  page  193.   (Original 
published  in  1758;  translated  into  German  in  1760.) 

2.  John  Eeinhold  Forster.  An  Introduction  to  L.ineralogy,  or 
an  Accurate  Classification  of  Fossils  and  Minerals,  viz., 
Earths,  otonos,  S;.lte,   Inflammable  and  Metallic  Substances* 
London  1768;  page  46. 


20, 


The  gradual  assembling  of  knowledge  concerning  bornite 
ie  easily  traced  in  the  increasing  numbers  of  Mineralogies 
published.  Kioherd  Kirwan,  who  first  approached  the 

problems  of  mineralogical  determination  from  the  ohemioal 

1 
side  states  in  his  Elements  of  Mineralogy  (London  1784) 

that  the  species  termed  Azure  Copper  Ore,  Kupfer  lazur, 
or  Kupfer  Malm  is  "cine  rail  zed  v/ith  sulphur  and  with  iron. 
Its  color  consists  in  various  shades  of  blue;  it  contains 
40  -  6O/£  of  copper,  20  -  30$  of  iron,  and  the  remainder 
sulphur;  the  poorer  it  is  in  iron,  the  richer  in  copper. 
It  has  been  by  many  confounded  with  indurated  mountain- 
blue."   In  an  edition  of  1810,  he  adds  that  the  mineral 
"effervesces  v/ith  nitrous  acid  importing  to  It  a  green 
colour;  does  not  immediately  give  a  blue  tinge  to  oaustio 
volalkall.  Its  specific  gravity  is  4.956  -  4.983  and 
effervescence  >vith  acids  prevents  all  possibility  of 
mistaking  it  -for  the  tarnished  ores  of  the  former  family 
Ccopper  pyrites}." 

The  first  distinct  analytical  results  to  be  pub- 
lished, appeared  in  1797  and  were  the  work  of  the  mineral - 

2 
ogist  Klaproth.  Material  from  Hitterdahl  in  Horway  and 

from  the  Friederike  Juliane  Mine  in  Eudelstadt,  Silesia, 
yielded  the  follOvvlng  re  suits  :- 

1.  Richard  Kirwan,  Elements  of  Mineralogy  -  London ,1784, 

2.  liartin  Heinrioh  Xlaproth,  Boitrage  zur  Chemischen 
Kenntniss  der  Mineralkbrper,  Vol.  II  ,~p.  281, (1797). 


21, 


Hitterdahl  Budelstadt 

Copper          69.50  $  58.  % 

Sulphur         19.  18 

Iron  7.50  19 

Oxygen          4.  5  % 

The  lo .  summation  of  the  copper,  iron  and  sulphur 
led  Klaproth  to  the  conclusion  that  the  variegated  colors 
of  the  mineral  were  due  to  oxygen r  which  was  believed  to  be 
present  in  sufficient  quantity  to  account  for  the  observed 
discrepancy.  The  peraistanoe  of  the  deep  color  of  the 
bornite  was  believed  to  be  due  to  the  thorough  abeorbtion 
of  oxygen,  similar  to  but  more  intensive  than  the  manner  in 
which  the  action  of  air  commonly  produces  an  iridescent 
tarnish  on  chaloopyrite .  For  such  early  work,  the  results 
are  remarkably  oloeo  to  the  usual  values  of  modern  analyses 
especially  if  the  "oxygen"  Is  added  to  the  sulphur  percentages* 
The  work  is  of  distinct  historical  interest  for  it  marks  the 
beginning  of  a  long  and  varied  series  of  analyses  and  their 
accompanying  controversies. 

The  name  Bmunt kupf e re rz ,  which  IB  commonly  used  in 

Germany  to-day,  was  piven  the  mineral  by  Werner  according  to 

1 
statements  in  the  books  of  several  of  his  followers. 


1.  Emmerling,  Uineralogy,  179G;  E.  Jameson,  ilineralogy,  1805, 
and  others.  Hintze  states  eroneouyly  that  the  name 
Euntkupfererz  was  given  by  Hoffman, (Min.  1816,36,110). 


22, 


The  term  Buntkupfererz  was  translated  Into  English 

1 

as  variegated  Copper  Ore,  "by  Jameson  who  gave  a  full  de- 
scription of  the  properties  of  the  mineral  in  his  Mineralogy 
published  in  1805.  Ho  stated  that  it  was  found  in  "beds, 
veins,  and  disseminated  In  rocks  of  different  formations,  and 
concluded  that  it  occurred  in  greatest  quantity  in 
primative  mineral  "beds,  according  to  the  tfernerlan  hypothesis 
of  the  formation  of  the  rooks  of  the  earth's  crust.  He  re- 
garded the  mineral  as  an  intermediate  species  between  oopper- 
glanoe  and  copper  pyrites,  and  "believed  that  it  occurred  as 
abundantly  as  the  former,  but  not  in  as  great  quantities  as 

the  latter. 

2 
Hauy  gave  a  description  of  bornite  under  the  name 

Cuivre  pyriteux  hepetloue,  and  stated  that  bornite  was  re- 
garded by  many  distinguished  mineralogists  to  have  originated 
from  chaleopyrite.  The  bright  and  varied  colors  often  ob- 
served on  the  surface  of  the  "cuivre  pyriteux"  were  regarded 

as  the  first  products  of  the  change. 

3 
The  name  Philllpsite  was  given  the  mineral  by  Beudant 

in  183T,  in  honor  of  the  mineralogist  and  chemist  li»R. 
Phillips,  who  had  described  crystal  forms  (cubes  and 
octahedrons)  and  analysed  Bpme  specimens  from  Boss  Island 


1.  Eobert  Jameson,  System  of  Mineralogy,  Edinburgh,  1803. 
Vol.  II.,  page  189. 

2.  C.  Hauy,  Traite  de  Mineralogie,  Paris, 1801. 

3.  Beudant,   Traite  de  Lllneralogie ,  Paris »183£»  Vol-II» 
page  411 . 


c  • 


S3. 


1 

In  Killarney,  Ireland.  The  name  is  oho  oldest  of  the  modern 

terminology,  nut  owing-  to  the  prior  usage  of  phillipsits  for 

a  member  of  the  zeolite  group,  it  has  not  been  retainer,  for 

. 
the  sulphiae,  ^looker  in  1839  created  tha  term  poikilopyrite t 

(from  ~n~oi/iAos  variegated,)  "but  even  though  it  may  justly 
olaim  priority  over  others  of  tha  modern  type,  it  is  rarely 

if  3ver  encountered  in  the  literature. 

3 
Dana  in  the  first  edition  of  his  mineralogy  (1837) 

called  the  mineral  "Pyrites  erubesoens",  alluding  to  Its 
liability  to  tarnish  and  to  assume  a  reddish  hue.  In  the 
edition  of  1850,  the  name  was  shortened  to  e rube site,  but 
the  term  was  discarded  in  the  later  editions,  due  to  the 
earlier  usage  of  the  name  nornite. 

The  oornmonly  accepted  namo,  bornite,  was  given  the 

4 
mineral  by  W,  Haidinger  in  1845  in  honor  of  Ignatius  von  Born 

5 
(1742-1791) .  In  a  letter  to  his  father,  Haidinger  wrote; 

"The  species  was  first  definitely  separated  from  chalao- 

1.  Phillips,  ;,iinerelogy  ,  1819,  page  2£2 . 

2.  Glooker,  ilinaralogie ,  1839,  page  328. 

3.  J.  D.  Dana,  System  of  Mineralogy,  1837,  page  408. 

4.  '".  Haidinger,  Handbuoh  der  bestioimenden  Mineralogie, 
.-n,  1845,  page  562. 

5.  Quoted  in  Hintse's  Handbuch  der  Mineralofle,  Bd.I,(1901), 
page  905  in  a  foot  note. 


24. 


cite  and  ohalcoi^yrite  in  the  classification  of  the  Royal 
Mineral  collection,  which  is  "being  newly  arranged  under  the 
direction  of  von  Born."  Tha  claim  that  this  was  the  earliest 
recognition  of  tha  distinot  character  of  the  species  may  be 
ouestioned,  "but  "by  general  usage  the  name  bornite  has  been 
firmly  established. 

Boll  shed  surfaces  07"  "hornite  were  first  made  by  H. 

1 
Baumhauer  in  1895,  to  study  the  properties  and  relations  of 

the  mineral  in  some  spoo linens  from  Chloride,  Hew  Mexico. 
He  observer7  that  the  grain  of  the  material  ?as  revealed  by 
etching  with  nitric  acid. 

Products  believed  to  be  bornite  have  been  obtainedartif ioiallyb: 
various  workers,  "but  as  the  homogeneity  of  thair  material  was 
established  only  in  a  rourh  v/ay,  the  results  have  little  more 

than  historic  value.  One  of  the  earliest  experiments  was  by 

£ 
Booking  in  1855.  36  g.  of  copper,  10  g.  of  iron  (re.-uoed 

from  the  oxide  by  hydrogen)  and  an  excess  of  sulphur  were 

melted  together  under  a  cover  of  cr.lt,  and  a  brittle  rerrulus 

was  obtained,  .vhich  appeared  like  bornite  on  the  fractured 

surface,  and  tarnished  in  moist  air  in  c.  i/irnil&r  way. 

By  analysis  it  contained:   Oopper  55.77.,  iron  15.9>t  sulphur 

25.9$ 

1.  H.  Baumhauer,  Uober  die  mikroskopishe  Beschaffenheit 
einer  Euntkupfererzeu  von  Chloride,  flew  Mexico.  Zeitsohr.  fur 
Kryjt.,  eto.  Vol.  10,  1885,  pa^o  447. 

£•  Inaug.  Diss.  GrOttingen,1855,  29.  Elntze »  Handbuch  der 

•or  .10,- MO,   Bd.  I. 


ittne 


25. 


1 
Marigny  in  1864  obtained  a  crystalline  aggregate  of 

material  resembling  borriite  by  melting  39  parts  of  pyrito; 
45  parts  of  oopper  shavings,  and  20  parts  of  sulphur  under 

a  cover  of  borax. 

2 

Doalter  produced  an  aggregate  of  small  cubes  correspond- 
ing to  the  formula  C\*3  Pe  3^  by  passing  hydrogen  sulphido  over 
a  mixture  of  Cug  0,  Cu  0  end  Fer^Og  at  a  low  temperature. 
(100-200  C)  without  melting  the  oxides.  Tho  material  agroed 
with  natural  bornite  in  oolor,  specif io  gravity,  analysis,  and 
crystal  form,  '^ith  a  mixture  richer  in  oopper,  some  oovallite 

was  obtained  with  the  bornite. 

3 
Beuss  observed  bornite  and  chalcopyrite  irregularly 

intergrown  in  a  blaok  slag  at  Hermannseifen  near  Trautonan  in 
Bohemia. 

In  tho  hot  springs  of  Bourbon  I'Arohambault    In  the 
Department  Allier,  rornan  coins  were  observed  altered  to 
bornite  and  ohaloopyrite;  at  Bourbonne-les-Bains  in  the  De- 
partment Haute  -  Marne,  email  crystals  of  hornlte  were  found 

4 
on  old  coins  in  an  artificial  water  course.  The  forms  (100) 


1.  Me.ripny,  Compt.  rand.,  1864,58,967.  Hintse,  ibid. 

0.  Doelter,Ueber  d.  kunstl.  Darstell.  oiniger  Mineral. 
&»d.  -rruppe  d.  Sulphide,  u.  Sulphosalze,  Zeit.  f.Krist.  u. 
Llineral.,    Vol.  11,  (18S5-86),pp  36-38. 

3.  Labor?,  Marz  I860,  10,  40.  Hint^e,  ibid. 

4,  Daubroe,  Corapt.  rend.,  1875,j30,  461,  604;  and  81,  16-, 
834,  1008.  LaoroiXf  ilin.  de  Pranoe,  1897,  _^;  677.  Mntze.ibid, 


27, 


rropertJ33 


Crystal  forne-  Bornite  crystallizes  in  the  Isometric 
system.  Good  crystals  are  rare,  "hut  descriptions  of  material 
from  many  widely  separated  localities  have  established  its 
forms  with  fair  certainty.  The  cube  and  octahedron,  alone  or 
in  combination,  and  the  trapezohedron  are  the  forms  most 
commonly  described.  Dana  notes  the  following  forr.is  and  com- 

binations:-  (100)   (110);  (111)  (110)  (100);  and  (111)  (110) 

1 
(211).  E.  H.  Kraus  and  J.  P.  Goldaberry  observed  the  following 

additional  forms  in  material  from  Bristol,  Conn.:-  (211),  (322), 
(433),  (411),  (522),  (533),  (833).   Only  411  is  considered 
doubtful. 

Crystals  of  bornito  several  centimeters  in  diameter 
have  been  found  in  the  Tyrol.  The  descriptions  of  the  deposits 

in  which  they  occur  are  very  brief,  but  they  are  probably  of 

2 
the  Alpine  type  .  iVeinschenk  describes  crystals  3  1/2  cm.  in 

diameter,  on  which  the  form  (211)  dominates  with  (100)  less 

prominent.  G.  Gasser  obtained  crystals  from  the  Prospnitzeralpe 

which  measured  5.6  X  M-.^  X  3  cm.   Sealenohe«irons 

of  oalcite  and  nodular  particles  of  native  gold,  about  1.  ::jr.. 


in  diameter,  adherer  to  one  side.  The  crystal  forms  (211)  and 
(322)  were  observed.   In  the  Hof  museum  in  Vienna,  there  is  a 


1.  The  Chemical  Conipositi  on  of  bornite  and  its  relation  to 
other  eulpho -minerals,  A.<7.S.rVol.  37,  p.  539.  (1914). 

2.  (J.  Gnsser,   Dio  llineralien  Tirolq,  einschliesslich 
Vorarlbergs  und  der  Hohen  Tauern,  Innsbruck,  1913,  p.  112. 


28. 


specimen  of  bornite  from  the  same  locality  which  is  a 
regularly  formed  trapezohedron  about  4.3  cm  in  diameter. 
Its  measurements  yielded  a  new  form  for  bornite,  viz:  (553), 
With  it  is  associated  native  gold,  calcite,  and  albite. 

Other  physical  properties.  Bornite  possesses  an 

1 
octahedral  cleavage.  The  cleavage  is  rarely  seen  in  the  hand 

specimen,  however,  but  may  be  easily  developed  on  a  microacopic 
scale  by  pressure  with  a  blunt  point.   In  the  plane  of  the 
polished  surface,  series  of  straight  cracks,  usually  parallel 
to  two  or  three  directions,  or  less  commonly  to  four  directions, 
which  form  in  the  bornite  around  the  edges  of  crushed  areas 
resulting  from  this  pressure  are  undoubtedly  due  to  the  oc- 
tahedral cleavage  of  the  mineral,   (fig.    )  The  fracture 
on  a  coarse  scale  is  uneven,  but  under  the  microscope  it  may 
be  seen  to  be  finely  conch oidal.  Bornite  varies  notably  in 
brittleness.  Material  from  certain  localities  breaks  so 
readily  that  it  is  difficult  to  polish,  while  that  from  other 
deposits  may  be  almost  ductile.  The  hardness  of  bornite  is  3. 

According  to  A.  Sella,  the  specific  heat  is  0.1177  (calculated, 

.1 
0.1195)  •   It  is  a  good  conductor  of  electricity;  the  resistance 

2 
increases  with  the  temperature. 


1.  Breithaupt,  Berg  and  Huttenm.  ztg.,  1859 ,18,322.  Hintze, 
Loc.  cit. 

n 

2.  Beitrag.  zur.  Kenntniss  der  sp.  Warrae  der  Min.,  Hach  d.k. 
Gas.  d.  ysriss.  zu  Gottingen,  1891,  10,   211-322;  also  Zeit.  f. 
Kryst.u.min.,  22,  180. 

3.  Beijerinck,  H.  Jahrb.,1897,  Beil.  Bd.,11,  437;  Hintzi,ibid. 


so 


£9, 


Bornlte  occupies  a  place  "below  marcasite,  chaloopyrlte, 
enargite,  oovellite,  and  pyrite,  and  above  galena,  chalcocite, 
hematite,  oUprite,  metallic  copper,  and  sphalerite  in  the 

eleotro -chemical  ueries  of  the  sulphides  and  oxides  determined 

1 
ty  V.  H.  Gottsohalk  and  H.  A.  Euehler.  Measured  against 

copper  v/ire  in  distilled  water,  bornite  v/as  found  by  them 
to  assume  a  charge  of   0.17  volts. 


1.  Oxidation  of  Sulphides  (Second  Paper) ?  Loon.  Geol., 
Vol.  7,  p.  31.   (1912), 


so. 


1 

Chemical  Properties. 

Before  the  blow-pipe  on  oharcoal  In  tho  reducing 
flame,  bornite  melts  to  a  little  raagnetlo  bead  with  a  gray- 
red  fracture  ;  with  soda  to  a  oopper  bead.  In  the  open  tube, 
sulphurous  fumes  but  no  sublimate  are  given  off;  in  a  closed 

tube,  a  weak  sublimate  of  sulphur,  Bornite  dissolves  with 

2 
efforvoooonoe  In  nitric  acid,  dilute  or  concentrated  with  the 

oeparation  of  sulphur.  It  IK  soluble  in  potassium  cyanide 

* 
solution,  whereby  it  may  be  separated  from  pyrite,  chal- 

oopyrite,  uagnetite  and  lollingltc.  It  precipitates  silver, 
often  in  crystals,  froia  cool  acid  silver  sulphate  solution* 
On  the  polished  surface,  nitrlo  aoid  changes  the 
normal  pin&ish  brown  color  of  the  mineral  to  a  golden  yellow. 
It  reacts  with  effervescence,  leaving  a  surface  etched  in 
fin<i  lines.   Stching  with  potassium  cyanide  produces  a  brown 
surface,  with  an  etch-pattern  similar  to  that  produced  by  nitrlo 
aoid.   Dilute  reagents  produce  coarser  etch-patterns  than 
concentrated  solutions.  With  both  potassium  cyanide  and  nitric 
acid,  the  surface  between  the  cracks  tends  to  arch.  Fre- 
quently little  pita  are  formed  by  the  blocks  springing  out. 
( Pig,  115).  The  change  is  probably  due  to  tension.  Some  speo- 
imene  of  bornite  show  it  to  a  much  greater  degree  than  others. 
Boiling  ferric  chloride  solution  (  5f? )  etches  the  polished 

1.  Summarized  in  part  from  Dana's  Mineralogy  and  Hlntze's 

:.   .  Min. 

2.  Richard  Kirwan,  Loc.  cit. 

?,  Lemberg,  Zeltschr.  d.  sool.  Ges.,  1900,  52, 


31, 


1 
surface  even  better  than  the  reagents  mentioned. 

Tho  etch-pattorna  produced  by  the  action  of  both 
nitric  acid  and  potassium  cyanide  on  the  polished  surface  of 
a  bornite  srain  most  cornraonly  consist  of  one  set  of  pertsistent 
and  closely  spaced  cracks  with  the  surface  between  them  broken 
into  imperfect  brick-like  blocks  by  short  cracks  roughly  at 
right  angl'     (Fig. 157).  In  certain  orientations  instead 
of  the  one  «tror.         -reaker  direction,  three  sets  of 
feeble  cracks  develop  which  yield  a  pattern  of  short  olosely- 

:;ed  lines  bre-tking  the  field  into  small  rhombs  or  triangles. 
When  three  sets  of  ahort  cracks  form,  they  are  parallel  to  the 
cleavage  directions  of  the  bornite;  where  one  strong  direct- 
ion of  tho  etch  pattern  is  dominant,  the  cracks  are  usually 
not  parallel  to  the  cleavage  directions. 


1.  Joseph  Murdoch,  The  MicrosoopioaiDetorrnination  of  the 
Opaque  Minerals,  Wiley  and  Sons,  New  York,  1916,   p.  65, 


32. 


The  Composition  of  Bornite. 

The  use  of  the  metallographlo  microscope  In  the  study 
of  sulphide  ores  In  recent  years  has  made  It  apparent  that 
pure  bornite  In  sufficient  quantities  for  analytical  purposes 

is  extremely  rare.  Material,  apparently  homogeneous  when 
examined  only  with  the  hand  lene,  is  often  found  to  contain 

surprisingly  large  quantities  or  ohalcooite.,  chaloopyrlte  or 
other  impurities  when  the  polished  surface  is  studied  micro- 
scopically. Crystalline  bornite  is  usually  found  to  be  aa 
impure  as  massive  bornite,  and  is  more  deceiving,  for  the 

replacing  sulphides  are  frequently  concealed  In  the  heart  or 

1 

basal  portions  of  the  crystals.   Consequently  it  is  not  sur- 
prising that  there  is  little  agreement  among  the  analyses  by 
the  earlier  mineralogists,  who  accepted  the  p;ood  crystalline 
form  of  the  mineral  as  the  sign  of  its  homogeneity. 

The  earliest  attempts  to  determine  the  composition 
of  bornite  wete  In  1?^  by  Kirwan  and  179?  by  Klaproth.  Their 
results,  which  are  of  historical  value  only,  have  been  mentioned 

on  preceding  pages.  The  formula  Cuj  Fe  83  ,  which  is  given  in 

2 
most  text-boofcB,  was  deduced  by  Plattner  in  1S>39  from  an 

analysis  of  crystalline  bornite  from  Cornwall.  This  was  fol- 
lowed in  the  same  year  by  in  inalyfiis  of  another  Cornish  spec- 
imen by  Varrentrap1,  while  a  third  by  Chodney,  appeared  in  the 
same  journal  in  1&3M-.  Subsequent  studies  have  confirmed  their 
analytical  results  In  a  rough  way,  but  have  shown  that  similar 

material  from  Cornwall  is  extremely  impure,  usually  with  chal- 

5 
oopyrite. 

1.  Joseph  Murdoch*  Op.  o It.,  Page  36  and  Frontispiece,  Pig.  1. 

2.  Plattner,  Pogg.  Ann,  14-7 1  351.  1839. 

3.  Varrentrap,  ibid. ,  p.  372. 

)L      lfV> /%<*>•,<*•»     4  ,  -I  A 


.     :  '  •   .  •    [ 


' 

rtB    CIC- 

. 


lie:  I 

IB  t 


35, 


The  analyses  are  as  follows: — 


I      II 

III 

IV 

V 

VI 

Cu 

56.76   5S.20 

57.S9 

57.71 

57.65 

55.  5« 

Pe 

lif.«M.   1M..S5 

1M-.94- 

13.  89 

15.11 

16.36 

S 

2S.2H-   26.9* 

26.  K^ 

27*17 

26.^6 

2S.06 

99.£ft-  100.02 

99.67 

9S.77 

99.25 

100.00 

I.  Condona  Uine,  Cornwall.  Plattner. 
II.  Cornwall.  Varrentrap. 

III.  Redruth,  Cornwall.      Chodney. 
IV.  &V.  Cornwall.  Harrlngton.(  Bornite  oontralning 

^haloopyrite,  visible  with  a  hand  lens). 
VI.  Calculated  for  CUz  Fe  Sz. 

As  Harrington  points  out  the  oomViinatlon  of 

CUB  Pe  83     and 

Ou  F.e  Sp     yields 

2CU*.  Pe  S^   ,  which  is  a  mixture  containing 
73,2$  bornite  and  only  26. 8fi  chaloopyrite ,  assuming  the 
first  formula  to  be  correct  for  bornite. 

In  1S75  Cleve  recognized  that  few  published  analyses 
of  bornite  agreed  with  the  formula  Cuj  Pe  83  .  Results  of 
several  analyses  done  under  his  direction  lod  to  the  for- 
mula Cu;j  ?e  84.  ,  but  from  others  more  complicated  formulae 
were  derived  which  forced  hin  to  the  conclusion  that  there 
were  several  forms  of  the  mineral  , 


34, 


The  variouf?  speculations  of  the  following  years  are 
of  little  value,  as  the  analytical  evidence  on  which  they 

were  based  is  questionable,  A  long  lisjc  of  analyses  is 

1 
given  in  Hintze's  Handbook  fur  Mineralogie. 

2 
In  1903,  8.  S.  Harrington  published  a  series  of 

analyses  of  bornite  from  various  Canadian  localities,  and 
round  that  his  results  agreed  closely  with  the  formula 
Pe  Sjj..  The  uniformity  of  his  results  indicates  that  the 
material  was  carefully  selected,  but  it  was  examined  only 
with  the  hand  ions,  which  oan  hardly  be  depended  upon  in 
general.  Hi;:  results  are  as  follows:  — 


I 

II 

III 

IV 

V 

VI 

CU   63.55 

6^.75 

62.73 

63.34- 

63.15 

63.27 

P0   10.92 

11.25 

11,05 

10.53 

11.25 

11.15 

s   25.63 

25.39 

25.79 

25.5* 

24-.  55 

25.55 

Insoluble  —  — 

.30 



.3* 

.24- 



100.10    99.75     99.57   100.09    99.55   100.00 

S.6  at 

15°  0       5.055        5.055  5.090         5.029  

I.  Harvey  Hill,  P.  Q. 

II.  Prince  Mine,  Ontario. 

III.  Dean  Channel,  Howe  Sound,  B.   C. 

IV.  Copper  lit.,   S inilfcameen  District,  B.   C. 

V.  Texada  Island,  B.  C. 

VI.  Calculated  for  Cu,-  Fe  84.  ,   for  coraparison. 

1.  .   914-916. 

2.  Loo.   cit. 


. 

..:  ' 
• 


ft* 


35, 


Ory:;t/uil::o''.  Somite,  (partly  nasHl^o,  partly  in 
rhonbio  dod'ioahodrons )  i"Vo~i  Bristol,  Conn,  was  analyzed 
as  follow?.:  — 

Ou      63.21J- 

Fe       11.20 

3       25.  p1*- 

99.9* 

Harrington  concludes  very  justly  from  these  data 
that  tho  composition  oi'bornite  is  correctly  expressed  by 
the  formula  Cu^  ?e  Sq.,  and  that  the  divergence  of  analyses 
in  the  pant  vas  due  to  the  impure  nature  of  the  material, 
The  question  however  was  reopened  by  £•  H,  Kraus 
J,  P.  Goldsberry  in  191*4-.   Two  analyses  wers  made  of 
crystalline  bornlte  from  Bristol,  vrhich  are  as  follows;  — 

Average 

Gu  65.M-2  65.91  65.665 

Pe  9*74-  9,67  9.705 

s  2M-.79  2*J-«5.1.  2^.656 

99.95  100.09  100.020 

S.o.  at  ordinary  temperatures     5.0S6 
Their  two  analyses  yield  the  fori.iula  Cu-j^  Peg  Sc,. 
Pieisos  of  the  sane  material  from  Bristol  as  was  analyzed  by 
Harrington   vero  obtained  ann  his  results  checked.     Based  on 
thiB  evidence,   and  nuinoroua  diver.c'inc  'ui'ilyBes  culled  from 
tho  literature,    the  authors  construct  an  elaborate  series 
of  minerals  v/ith  the  general  formula  Cux  ?&Q  Sy,  v/here 


36, 


y  -  X/2  f  3.  The  series  ranges  from  Cu  Pe  S2  (  ohalcopyrite  ) 
through  CUy^  Peg  8^  with  ohalcocite  as  the  final  limit* 
Each  member  differs  from  the  preceding  by  the  radical  Qu^  S. 

The  bornite  analyzed  was  examined  by  ordinary  methods 
and  considered  pure,  A  metallographlc  examination  of  the 
material  actually  analyzed  was  not  made,  but  material  from 
the  same  group  of  crystals  was  studied  In  this  way  later  and 
found  to  be  pure.   Similar  specimens  from  Bristol,  however, 
hav    n  found  to  contain  large  amounts  of  ohalooolte, 
consequently  thoir  high  copper  values  are  not  above  suspicion* 

This  morpho  tropic  series  postulated  by  Kraus  and 
Goldoberry  has  been  criticized  by  A*  P.  Rogers}  who  suggests 
that  the  variation  In  composition  of  bornite  may  be  ex- 
plained as  the  result  of  solid  solutions  of  ohalooolte 
i  .  bornite  of  the  formula  Ou*  Pe  8*  .   The  two  analyses  of 
Kraus  an'  Goldsberryare  accepted,  but  no  new  evidence  is 
brought  forward  except  a  triangular  diagram  upon  which  nu- 
merous -analyoeB  givon  in  Hintze  are  plotted.  The  dotu  form 
an  irregular  oval,  the  longer  axis  of  which  is  roughly 


parallel  to  the  line  Cux  Pe  S(  x  f^,  but  the  coincidence  is 
not  striking.   The  most  significant  point  brought  out  by  the 
diagram  is  the  clustering  of  dots  around  the  point  corres- 
:  ending  to  tho  formula  Ou^  Pe  814..  The  obvious  significance 
of  tho  fact  IB  disregarded  by  Rogers,  who  merely  states  that 
it  may  be  explained  as  the  value  for  the  average  solubility 
of  chalcooite  in  bornite, 

1.  Science,  New  Series,  Vol.  4-2,  p.  3#6,  1915* 


37. 


1 
3,  T.  Wherry  in  a  discussion  of  Rogers'  paper 

differs  from  this  interpretation,  and  shows  that  the  variation 
in  the  composition  of  bornite  nay  toe  explained  equally  well  by 
assuming  Cu^  ?e  8^  as  the  formula  of  "normal"  bornite,  with 
the  variation  in  one  direction  caused  by  the  presence  of 
chalcopyrite  or  pyrite  in  solid  solution,  and  in  the  other 
direction  by  the  presence  of  chalcoolte.  He  does  not  favor 
the  idea  of  solid  solutions,  however,  but  believes  that  the 
variations  in  apparently  pure  material  are  due  to  eubmicrosoopio 
inclusions  of  ohalcopyrite,  pyrite  or  ohalcoolte.  The  limit 
of  microscopic  visibility  (  about  0*001  ma, )  is  determined  by 
the  wave  length  of  light,  and  there  is  no  reason  to  assume  that 
the  size  of  inclusions  stops  at  this  point.  The  suggestion 
is  a  valuable  one  for  there  is  evidence  which  will  be  presented 
lator,  that  submicrosooplo  ohalcopyrite  may  develop  in  bornite. 
Even  In  such  cases,  hov/ever,  the  Impure  nature  of  the  boraits 

w 

is  revealed  by  the  abnormal  color  on  the  polished  surface, 
and  it  is  vary  probable  that  the  variations  in  the  analyses  pre- 
viously mentioned  hare  been  largely  due  to  Included  material 
which  could  have  been  easily  detected  under  the  microscope. 
It  la  vory  improbable  that  mibralorosoopic  pyrite  is  of  Im- 
portance, The  difference  in  hardness  of  bornite  and  pyrite 

^9  even  small  grains  of  the  latter  very  apparent  when  a 
bornite  specimen  is  poliwhed. 

1,  Science,  Vol.  M-2,  II,  s.,  p,  570,  1915. 


38. 


The  controversy  over  the  composition  of  bomlte, 

however,  may  bo  regarded  as  settled  by    malysea  of  the  mineral 

1 

in  the  Geophysical  Laboratory  in  Washington,  by  Dr.  E.  T.  Allen* 

Polishod  surface a  of  the  specimens  were  carefully  studied  in 
all  cases,  and  the  material  found  to  bo  practically  free 
fron  impurities,  with  the  exception  of  one  specimen  from  North 
Carolina  which  contained  a  little  ohalcocite.  Applying  a 
correction  in  this  one  case,  tho  results  are  renar&able  uni- 
form, and  confirm  Harrington's  analyses  closely.  The  for- 
mula Cu^  pe  814.  is  indicated  with  a  high  degree  of  certainty. 
Dr.  Allen1 «  analyses  are  given  below. 

2 
Analyses  of  Natural  Bornltee 


Local- 
ity 

Superior, 
Arizona 

Urn- 

Costa 
Rloa 

Bristol 
Conn. 

,  Oullf  ord 
Co.  ,N.C. 

Messina 
Trans- 
vaal 

Cal 

Cu^ 

•  for 

re  s^ 

Cu 

rk 
s 

Pb 

62.99 
11.23 

25.  5S 
.10 

63.19 
11.31 

none 

63.08 
11.22 
25.5M- 
none 

63.26 

63.90 
10.79 
25.17 
none 

62.2M- 
11.12 
25.54- 

63. 
11. 
25. 

33 
12 

55 

A/» 

none 

.02 

none 

'. 

none 

99.90    99.96  99. 


99.«6   99.90  100.00 


at 


25° 
Ifater  at  25°  5.076 


5.076  5.052   5.079    5.103   5.09H- 


Mineral  at 

25°  0 
fater  at  M- 


5.061         5.061     5.037       5.061J- 


5.079 


1.  Composition  of  natural  bornlte,  A.  J.  S.,  Vol. 

1916,  Pp.  M-09-^13. 

2.  E.  T.  Allen,  Loc.  oit.,  p. 


39. 


Dr.  Allen's  paper  concludes  as  follows;  — 
For  the  sake  of  oompletoness,  we  may  Include  here  a  recent 
analysis  of  the  bornito  from  Virgilina,  Virginia,  by 
Ohase  Palmer.    Palmer's  material  was  examined  metallograph- 
ically  and  it  la  noteworthy  that  the  principal  impurity  was 

Pound      Cal.  for  Gu^  Pe  Sij. 
CT!  62.50         63.33 

Fe  11,64-         11.12 

S  25.  '10        :  25*55 

99.5^         100.00 

ohaloopyrite.  Though  the  results  are  in  fair  agreement  with 
CUc  Fe  54.,  'the  relatively  high  proportion  of  iron  suggests 
the  presence  of  a  small  quantity  of  ohaloopyrite  r/hich  in 
preparing  the  sarjple  for  n-tudy  has  escaped  detection.  ' 

"Finally,  observations  of  two  physical  properties  of 

2 

bornite  confirn  the  chemical  evidence.  Murdoch  states  that 

•the  raineralographic  examination  of  polished  surfaces  of 
bornlte  from  at  least  30  different  localities  has  revealed 
only  an  exceedingly  slight  variation  in  color  and  practically 
none  in  niorochemical  behavior.  While  reli-ince  on  density 
alono  as  a  criterion  of  purity  is  unsafe,  the  determinations  in 
the  table  [p.  3S   ire  oonf  imatory.  All  the  determinations 
aro  in  good  accord  (  5«06l  to  5.079)  except  that  on  the  Costa 
Rica  Bp^ctoen;  and  while  it  may  be  possible  that  we  have  here 


1.  J.  Wash.  Acad.  Sol.,  5,  351,  1915. 

2.  Loc.  ^it.,  p.  35. 


' 


40, 


-mother  crystalline  form  of  the  sane  composition;  the  low 
value  for  this  bornite  is  reasonably  accounted  for  by  Dr. 
Morwin's  observations,  viz.,  that  under  the  microscope  it  has 
a  porous  appearance, 

•Aside,  then,  from  these  slight  variations  in  con- 
position  vhioh  are  so  common  throughout  the  mineral  kingdom, 
•wl  vrhich  are  'lue  to  foreign  admixtures  or  to  solid  solution, 
there  IB,  in  my  opinion,  no  satisfactory  evidence  that  natural 
bornite  la  variable  in  compos it ion,  or  that  it  is  even  of 
any  other  composition  than  that  expressed  by  the  formula 
Cu  Pe  Sij..* 

Still  later  investigations  now  about  ready  for  pub- 
lication by  the  chemists  of  the  Geophysical  Laboratory,  show 
that  Hynthetio  products  of  the  composition  CU5  Pe  S^.  agree 
in  all  respects  with  natural  bornite  whereas  synthetic  product* 
of  other  compositions  differ  more  or  less  notably  from  natural 
bornite. 


PAR  T _     II  I 


DiSSCEIPITIOBS       09       DEPOSITS 


41. 


PART  III 

DESCRIPTION?  OF  DEPOSIT0 
MAGMATIC-PNEUMATOLYTIC  DEPOSITS 
Qokiep.  Little  Namaaualand.  South  Afrio^ 

The  ohaloopyrite  and  bornite  deposits  in  the  vicin- 
ity of  the  town  of  Ookiep  in  Little  Kamaqualand,  Cape  Colony, 
South  Africa  have  been  considered  by  many  writers  to  be  of 

magmatio  origin* 

f 
This  interpretation  is  supported  by  0.  P.  Tolman, 

Jr.  and  A.  F.  Rogers1  who  hare  recently  published  the  results 

1.  A  study  of  the  Magmatio  Sulfid  Ores,  Leland  Stanford 
Junior  University  Publications,  University  Series,  1916, 
pp.  59-60. 

The  following  bibliography  is  given  in  Tolman  and  Rogers1 
paper: 

Delesae,  U.  —  Sur  lee  mines  de  cuivre  du  Cap  de  Bonne 

e,   Ann.  des  Mines,   «•  eerie,  8,  pp.  186-313, (1855). 
Wiley,  A.  —  Report  on  the  mineral  and  geological  struo- 
h  Hamaqu  Land,  Parliamentary  Report,  Cape  Town, 

Zerrener,  C.  —  Reise  des  Ingenieurs  A.  Thiers  nach  den 
3ergwerken  Namaqualande  in  Sud  Africa,  Berg,  und  Hut- 
tenm.  Zeitung,  1860,  pp.  41-44  and  53-54. 

Knopf,  A.  —  Uber  die  Kupfererzlagerstatten  von  Klein 
aaquaxand  and  Demaraland,  Neues  Jahrh.  f.  Min.  Geol.  u. 
Pal.,  1861,   pp.  613-550. 

Schenk,  A.  —  Die  Kupferenslagerstatten  von  Ookiep  in 
Hamaqualand,  Zeit.  der  deutsoh.  Geol.  Ges.  Verhand. 
Ges.,   pp.  53,  64-65,  (1903). 

Kunta,  J.  —  Copper  ore  in  Southwest  Africa,  Trans.  Geol. 
Soo.  South  Africa,  7,  pp.  70-76  (1904). 

—  Kupfererzvorkommen  in  Sudweatafrika,  Zeit. 
f.  prakt.  Geol.,  12,  pp.  199-303,   (1904). 

Ronalds  on,  J.  H.  —  Notes  on  the  copper  deposits  of  Lit- 

ind,  Trans.  Geol.  Soc.  South  Africa,  8,  pp. 
Io8-167,  (1905). 

„  «*  Stutaer,  0.  —  Magmatische  Ausecheidungen  von  Bornit, 
t.  f.  prakt.   Geol.,  15,  p. 371  ,  (1907). 

Rogers,  A.  W.  —  The  nature  of  the  copper  deposits  of 

lamaqualand,  Proceed.  Geol.  Soo.  Southwest* Africa, 
191o,   pp.  21-34. 


42, 


of  a  microscopical  study  of  a  suite  of  specimens  from  the 
district* 

Specimens  from  this  district  have  not  been  available 
for  study  in  connection  with  this  work,  but  ae  descriptions 
indicate  that  the  magmatio  origin  of  the  ores  is  more  definite- 
ly shown  than  in  the  case  of  any  of  the  bornito  deposits  of 
this  continent  a  brief  summary  of  their  important  features 
will  be  given. 

The  ore-minerals  ocour  in  a  great  variety  of  coarse 
grained  basic  intrusivee,  among  which  norite,  mica  diorite, 
augite  diorite,  diorite,  hypersthenite  and  anorthoeite  have 
been  described*  The  rooks  may  be  uniform  within  a  single 
body,  or  two  or  more  varieties  may  be  associated*  The  contacts 
in  these  oases,  however,  are  sharp,  and  interpreted  as  evidence 
of  differentiation  before  intrusion.  Dikes  and  sill*  are  the 
commonest  forms,  but  horizontal  sheets,  stocks,  pipes  and 
irregular  branching  bodies  have  been  described,  and  according 
to  A*  W.  Rogers  344  of  them  have  been  mapped*  (1916)*  The 
ore  occurs  in  lenticular  shoots,  usually  with  the  greatest 
dimension  in  a  horizontal  position*  The  ore-bodies  ocour 
in  various  parts  of  the  intrusives  and  in  some  oases  break 
into  the  wall-rock,  which  is  the  fundamental  gneiss  of  South 
Africa* 

The  ore-minerals  are  magnetite, ilmenite,  hematite, 
pyrrhotite,  bornite  and  chalcopyrite.  *'rom  the  microscopical 
studies  of  Tolman  and  Rogers,  it  is  established  that  the  sul- 


43, 


phides  are  without  question  later  than  the  rook  silicates. 
A  small  amount  of  euhedral  magnetite  occurs  in  the  earlier 
rock  minerals,  but  it  is  believed  by  the  authors  to  be  of 
replacement  origin.  The  feldspars  appear  to  be  more  easily 
replaced  than  the  pyroxene,  and  the  ores  are  more  closely 
associated  with  them.  Biotite  is  very  susceptible  to  the 
attack  of  the  sulphides,  and  is  usually  replaced  by  them  a- 
long  its  cleavages. 

The  rocks  from  many  of  the  mines  show  almost  no 
traces  of  the  alteration  products  which  commonly  accompany 
sulphide  ores.  Anthophyllite,  olinozoieite  and  chlorite 
are  mentioned,  but  are  not  of  general  occurrence.  Anthophyl- 
lite, cutting  bornite  and  chalcopyrite  in  a  vein-like  band, 
and  penetrating  the  sulphides  from  the  borders  of  grains, 
is  described  by  Tolman  and  Rogers,  and  the  relations  are 
clearly  illustrated  by  their  excellent  photographs.1  Chlo- 
rite is  said  to  be  later  than  the  sulphides.  One  of  their 

p 
photographs   shows  a  ve inlet  of  chlorite  breaking  opaque 

minerals. 

The  sequence  of  the  ore-minerals  is  not  very  clearly 
shown.  From  the  shape*  of  grains,  Tolman  and  Rogers  state 
that  it  is  probable  that  the  bornite  was  formed  in  part  by 
the  replacement  of  magnetite.  There  is  good  evidence  that 
ohalcopyrite  is  later  than  pyrrhotite3,  and  a  suggestion 

1.  Op.  cit.,  Plate  XIII,   Fig.  55  and  Plate  XV,  Fig.  63-63. 

2.  [bid,,  Plate  XVI,   Fi£.  65. 

3.  Tolman  and  Rogers,   Op.  cit.,  Plate  XVI,   Fig.  64. 


44, 


that  pyrrhotite  has  in  part  been  formed  by  the  replacement 

1 
of  magnetite*   The  relation  between  the  bornite  and  the 

ohalcopyrite  is  not  stated*  Chaloopyrite  gashes  cutting  the 
bornite  are  described,  and  are  believed  to  be  the  products 
of  post-magmatic  readjustment a. 

The  Ookiep  ores  for  tha  most  part  are  evidently  of 
the  same  origin  aa  many  sulphide  ores  elsewhere  which  are 
commonly  classified  as  magmatio*  The  ore-minerals  are  later 
than  the  original  rock  silicates,  but  this  is  commonly  found 
to  be  the  case  in  ores  of  this  sort*  Alteration  products 
characteristic  of  hydrothermal  ores  are  lacking  or  only 
slightly  developed*  The  conditions  under  which  the  bornite 
in  the  Ookiep  ores  was  produced  may  be  regarded  as  more 
nearly  magmatic  than  in  the  case  of  any  other  bornite  deposit 
known*  If  the  Ookiep  ores  are  not  strictly  magmatic,  they 
are  at  least  the  closest  approach  to  magmatic  bornite  ores 
that  have  been  described. 


1.   Ibid.,  Plate  XVI,   Fi£.  67, 


45. 


La  Plour  Mountain.  Danville  District t 
Ferry  Co..  i?aahington« 

INTRODUCTION 

Bornlte  oros  of  unusual  interest  are  exposed  in  several  pros- 
pects on  the  slopes  of  La  Fleur  ',rt,,  Washington,  a  few  miles  south 
3f  the  international  boundary  and  about  10  miles  southwest  of  the 
bown  of  Grand  Forks,  B.  C.  The  ores  have  not  been  described  pre- 
viously. The  only  information  available  concerning  the  general  ge- 
ologic relations  is  to  be  found  in  the  reports  of  the  Canadian  Ge- 
ological Survey  ,  which  deal  with  the  Boundary  District  immediate- 
ly to  the  north.  The  nearest  mine,  the  Lone  Star,  a  small  proper- 
ty of  the  British  Columbia  Copper  Company,  is  about  3  miles  distant, 
md  the  large  deposits  of  pyritio  ore  near  Phoenix,  worked  by  the 
Jranby  Co. ,  are  ten  miles  or  more  across  the  boundary.  The  miner- 
ilization  of  these  deposits,  however,  is  distinctly  different  in 
jharacter  from  that  of  the  bornite  deposits,  and  there  is  probably 
10  genetic  relation  between  them. 

The  claims  on  La  fleur  Mt.  have  been  developed  only  to  a  very 


1.  E.  ff.  Brook,  Summary  reports  of  the  Director  of  the  Geolog- 
ical Survey  of  Canada  for  1901  and  1902. 

R.  A.  Daly,  Seol.  of  the  North  American  Cordillera  at  the 
19th  Parallel,  Part  I,  p.  277,  (1915). 

0.  3.  LeRoy,  The  Geology  of  the  Phoenix  IHning  District, 
iemoir  21,  C.G.S.,   1912. 


•  •  . 


i 


46, 


slight  extent  "by  a  few  shallow  shafts  or  short  adits,  A 
little  ore  has  been  mined,  and  shipped,  but  no  noteworthy 
work  has  been  done. 

OEOLQaiCAL  DELATIONS. 

She  older  rocka  of  the  mountain  form  a  aeries  of  vari- 
ous sediments,  of  which  limestone  beds  and  some  altered  por- 
phyries are  prominent  members.  ?rom  the  mapping  on  the  Cana- 
dian side  of  the  line,  it  is  possible  that  the  sedimentary 
rooks  may  be  members  of  the  Attwood  series,  argillites, 
quartzites  and  limestones,  stated  to  be  of  Carboniferous  (?) 
age.  A  diorite  porphyry,  now  altered  in  places  to  a  somewhat 
sohistose  rook  containing  abundant  biotite  and  shreddy  araphi- 
bole,  is  associated  with  the  sediments,  but  neither  its  exact 
relations  to  them  nor  the  form  of  its  bodies  could  be  detect- 
ed. The  original  rock  possessed  small  phenocrysts  of  andeslne, 
oligoelase  and  biotite,  set  in  a  microgranitic  ground  mass  of 
the  same  mineral,  in  which  there  is  also  a  little  orthoolase 
and  fine  shreds  of  amphibole. 

The  older  rocks  of  the  mountain  are  intruded  by  an  ir- 
regular mass  of  gabbro.  The  rock  varies  from  dark  varieties, 
in  which  hornblende  becomes  prominent,  to  lighter  types  in 
which  feldspar  predominates .  In  the  commonest  form,  the  rock 
possesses  a  medium  to  fine  grained  texture,  and  is  composed 
chiefly  of  acid  labradorite  (Ab5  A^)  and  colorless  to  pale 


. 


47 


green  augite.  Hornblende  is  usually  present,  apparently 
formed  In  part  at  the  expense  of  the  pyroxene.  In  places, 
the  rook  becomes  of  dioritic  character.  The  mass  of  the 
rock  is  cut  by  numerous  small  seams  of  pegraatitic  material, 
nearly  always  feldspathio,  with  little  or  no  quartz.  They 
are  usually  less  than  one  inch  in  thickness. 

The  gabbro  is  out  by  dikes  of  coarse  syenite,  averag- 
ing about  four  feet  in  width,  but  occasionally  much  wider. 
In  places,  the  rock  becomes  distinctly  pegmatitic  in  charac- 
ter, with  the  orthoclase  in  crystals  two  or  three  inches 
long.  A  sub-parallel  orientation  of  the  feldspar  prisana 
is  usually  prominent.  It  suggests  a  flow  structure.  The 
chief  mineral  of  the  syenite  or  the  syenite-pegmatite  is 
orthoclase •  Under  the  microscope,  albite  is  found  to  be 
common  also,  usually  in  smaller  grains  around  the  margins 
of  the  coarser  orthoclase,  and  to  a  slight  extent  in  mi- 
croperthitio  intergrowths.  Muscovite  is  the  only  other 
mineral  prominent  in  the  syenite*  It  occurs  as  medium 
grained  flakes  of  slightly  greenish  tinge  when  seen  with 
the  unaided  eye*  The  mica  is  closely  associated  with  the 
albite,  and  is  usually  a  constituent  of  the  zone  of  finer 
grains  surrounding  the  coarse  orthoclase  crystals.  Both 
the  albite  and  musoovite  are  in  part  later  than  the  or- 
thoclase. Other  constituents  are  of  little  quantitative 
importance*  Titanite  in  sharp  crystals  is  not  uncommon. 


48, 


and  there  is  a  little  apatite.  7/here  in  contact  with  the 
altered  porphyries  of  the  older  aeries,  a  little  augite 
sometimes  developes  in  the  dike  rock.  Other  modifications 
with  respect  to  the  wall  rocks  are  closely  related  to  the 
processes  oi'  the  mineralization  and  will  be  described  in 
that  connection.  The  dikes  are  in  a  north-south  zone  and 
may  bo  traced  about  a  mile,  although  no  single  dike  is  con- 
tinuoiis  for  that  distance. 

The  latest  igneous  rock  is  a  gray  diorlte  por- 

• 

phyry,  which  forms  a  broad  dike  cutting  all  other  formations. 
It  is  fresh  and  unmineralized.  and  in  striking  contrast  to 
the  older  and  more  or  less  altered  rooks  of  the  mountain. 

\        m  Aj&*^  \ 

V   ]fj^^ 

MUTSRALI ZATIOH 

Ores  in  the  Syenite  Dikes.  The  mineralization 
is  closely  associated  with  the  syenite  or  syenite-pegmatite 
dikes.  The  sulphides,  bornite  and  chaloopyrite^ are  most 
abundant  in  the  dikes  themselves,  but  where  the  dikes  out 
the  gabbro,  the  sulphides  penetrate  the  wall-rock  along 
fine  seams  or  occur  disseminated  through  it  to  a  slight  ex- 
tent. The  mineralization  in  the  dikes  however  is  not  con- 
tinuous and  it  is  only  locally  that  the  sulphides  are  suf- 
ficiently abundant  to  constitute  an  ore. 

The  ore-minerals  corrode  the  orthoclase,  albite 
and  Muscovite,  but  with  the  exception  of  an  albite  rim  which 


. 


' 


49, 


often  ia  present  between  the  ore-minerals  and  the  orthoolaae, 
there  la  no  other  important  alteration  of  the  rook-minerals. 
A  little  fine-grained  musoovite  or  serioite  is  sometimes  de- 
veloped in  the  albite,  and  later  oaloite  replaces  the  feld- 
spars to  a  slight  degree,  but  the  absence  of  alteration  pro- 
ducts which  usually  accompany  sulphide  mineralization  is  an 
outstanding  feature. 

Mineralization  in  the  Wall-rocks*  Where  the  dikes  cut 
the  limestone  members  of  the  sedimentary  aeries,  a  garnet 
rook  is  formed  composed  chiefly  of  ondradite,  albite  and  oal- 
cite,  with  a  little  diopside,  titanite  and  apatite,  The  rock 
contains  numerous  small  cavities  between  the  crystals  of  its 
constituents,  and  a  scattering  of  chalcopyrite  grains.  Gar- 
net is  also  abundant  in  ono  part  of  the  syenite  dike  on  one 
of  the  claims  on  the  east  end  of  the  mountain,  and  is  probably 
to  be  attributed  to  the  influence  of  limestone  on  the  magma* 
The  garnet  is  poikilitio  with  albite,  and  probably  crystal- 
lized later  than  the  feldspars.  The  mica  in  this  part  of  the 
dike  is  biotite  (deep  green  parallel  to  c  ,  pale  yellowish 
green  to  colorless  perpendicular  to  c  ) ,  and  the  colorless 
muscovite,  abundant  elsewhere,  is  in  small  flakes  and  quite 
subordinate. 

Where  the  mineralization  has  penetrated  ths  gabbro, 
there  is  more  pronounced  rock-alt oration.  There  is  a  notable 
Increase  in  hornblende  near  the  dikea,  and  also  along  the  small 


50, 


seams  of  feldspathic  pegraatitio  material  which  cut  the  rook* 
The  sequence  of  minerals  is  well  shown  in  one  thin  section 
which  includes  a  portion  of  the  edge  of  one  of  these  string- 
ers. The  pegmatitic  material  is  largely  orthoclase  and  al- 
blte,  in  inicroperthitic  relations  for  the  most  part;  near  the 
margins  garnet  is  slightly  developed.  In  the  rock  nearest  th« 
seam,  hornblende  is  most  abundant.  In  places  it  possesses  a 
rim  of  deeper  green  than  the  heart  of  the  grain;  (pleocroism: 
deep  green  parallel  to  b;  yellow  or  pale  brown  parallel  to  a; 
light  yellow  or  yellowish  green  parallel  to  c).  farther  into 
the  rock,  pyroxene  (colorless  augite)  becomes  of  greater  im- 
portance and  io  the  chief  femic  constituent.  Kpidote  and 
chlorite  are  abundant  in  the  rock,  as  replacements  of  horn- 
blende, pyroxene  and  the  feldspars.  The  older  plagioclase  of 
the  rock  contains  abundant  sericite  and  calcite.  Apatite  and 
titanite  are  somewhat  more  abundant  in  the  rock  than  is  usual- 
ly the  case.  The  chief  opaque  mineral  in  this  particular  slide 
ia  magnetite,  which  is  later  than  the  pyroxene  and  feldspars, 
and  in  part  at  leaat,  later  than  the  hornblende,  for  all  the 
preceding  minerals  are  clearly  corroded  by  it.  It  frequently 
includes  small  apatite  grains. 

Ore-:.!inorala.  The  ore-minerals  are  chiefly  chalcopyrite 
and  bornite.  The  former  is  slightly  nore  abundant.  Magnetite 
ia  largely  confined  to  the  gabbro;  it  was  not  observed  in  ores 
from  the  syenite.  The  ore-minerals  in  the  dikes  are  later 


ae 


of;! 


51. 


than  the  rock-minerals,  as  has  been  stated,  although  probably 
not  very  much  later .for  there  are  so  few  alteration  products 
associated  with  then.  In  the  gabbro,  they  are  probably  con- 
temporaneous with  the  epidote  and  chlorite. 

The  chalcopyrite  and  bornite  are  usually  in  close  asso- 
ciation, and  in  part  probably  of  contemporaneous  origin.  The 
contacts  between  the  two  minerals,  as  shown  on  the  polished 
surface,  are  smooth  and  even,  with  few  irregularities  such  as 
tongues  or  veinlets  which  would  definitely  show  sequence.  In 
many  oases,  their  relations  to  each  other  would  be  little  al- 
tered, if  the  two  minerals  were  interchanged.  This  relation 

i 

i 

has  been  termed  the  mutual  b oundargr  ,  and  is  believed  to  be 

particularly  characteristic  between  minerals  of  the  primary 
sequence  (figure  67  ).  There  is  a  little  evidence,  however, 
of  corrosion  of  the  ohalcopyrite  by  the  bornite  in  some  places, 
and  it  is  probable  that  conditions  favoring  the  formation  of 
bornite  continued  after  the  deposition  of  chalcopyrite  had 
ceased,  which  resulted  in  a  slight  replacement  of  the  chalco- 
pyrite by  the  bornite.  This  interpretation  is  shown  more 
clearly  on  the  diagram  of  mineral  sequence. 

Magnetite,  and  a  small  amount  of  specularite  are  probab- 
ly earlier  than  the  sulphides,  but  their  relations  could  not 
be  determined  with  final  satisfaction  in  this  deposit. 

A  little  sphalerite,  galena,  and  tetrahedrite  were  ob- 


1.   L.  C.  Graton  and  I).  H.  McLaughlin,  Ore  deposition  and 
enrichment  at  Engels,  California*  tioon.  Geol.,  Vol.  XII,  1917,  p  17, 


52, 


served  as  small  spooks  on  the  polished  surfaces  in  the  midst 
of  the  more  abundant  sulphides. 

OXIDA2IC1I  A1TD  SilKICHI,!; 

Tho  deposits  occur  in  a  mountainous  region  of  mature 
topography  (Fig.  6  }  and  possess  no  noticable  gossan,  a  fact 
whioh  nay  be  attributed  to  the  present  vigorous  erosion  and 
preceding  glacial  scour  to  whioh  they  have  been  subjected*  A 
small  amount  of  linonite  and  malachite  occur,  but  the  sulphides 
outcrop  directly  at  the  surface  in  a  fairly  fresh  condition* 
A  small  amount  of  chalcoeite,  covellite  and  chalcopyrite  occur 
in  the  bornite,  and  are  undoubtedly  to  be  attributed  to  the 
processes  of  secondary  enrichment,  but  as  far  as  their  total 
copper  content  is  concerned,  they  are  of  little  Importance. 

She  chalcocite  is  in  fine  veinlets  or  rims  associated 
with  the  bornite.  It  follows  the  easy  channel-ways  afforded 
by  the  contacts  of  bornite  and  the  gaugue  or  rock  minerals, 
or  seeks  out  grain  boundaries,  seams  or  other  obvious  lines 
of  attack.  Covellite  is  for  the  most  part  the  first  stage  of 
the  alteration  of  chalcocite  to  malachite  and  usually  occupies 
an  intermediate  position  between  the  two  minerals.  It  dovel- 
opes  directly  from  bornite,  however,  to  some  extent.  2he 
veinlets  of  chalcooite  and  covellite  penetrate  the  primary 
ohalcopyrite,  but  the  latter  is  clearly  more  resistant  to  al- 
teration, and  they  do  not  develop  as  easily  as  in  the  bornite. 

The  bornite  in  the  neighborhood  of  the  chalcocite  and 
covellite  veinlets  commonly  contains  a  small  amount  of  chal- 


I*  $ 


-,  -    ' 
- 


tat 


53. 


oopyrite,  which  is  closoly  related  in  origin  to  the  secondary 
sulphides.  The  chaloopyrito  in  this  association  developes 
as  plates  in  the  bornite,  apparently  oriented  parallel  to  throe 
or  rarely  four  crystallographic  directions,  which  on  the  pol- 
ished surface  yield  the  reticulate  pattern  known  as  the  lattice 
structure.    In  some  grains  of  bornite,  the  plates  of  secon- 
dary chalcopyrite  are  so  thin  and  so  finely  spaced  that  they 
are  scarcely  visible  under  the  highest  available  magnifica- 
tions, and  it  is  justifiable  to  assuao  that  the  peculiar  yellow 
oolor  of  certain  areas  of  bornite  in  the  same  vicinity  is  due 
to  subraicrosoopic  chalcopyrite  of  this  sort. 

DISCUSSIOH 

The  possibility  that  the  syenite  and  syenite-pegmatite 
dikes  are  late  products  of  the  same  magma  which  produced  the 
gabbro  offers  a  problem  of  great  interest. v  The  small  seams 
of  pegraatitio  material  which  are  common  in  the  gabbro,  and 
which  would  ordinarily  be  accepted  es  its  pegmatitic  phases, 
are  closely  related  mineralogically  to  the  syenite,  and  are 
probably  of  the  same  origin.  The  greater  alteration  and  more 
abundant  mineralization  of  the  gabbro  along  the  ore-bearing 
portions  of  the  syenite  dikes  than  it*  the  case  where  other 
rocks  are  encountered  might  be  interpreted  as  an  indication 
that  the  gabbrp  was  still  heated  at  the  time  of  the  intru- 


54. 


sion  of  the  syenite,  and  in  a  receptive  state  to  react  with       // 
the  vapors  given  off.  There  is  no  evidence  against  the  view,  ^ 
and  although  the  arguments  in  its  favor  are  not  of  direct  or 
compelling  character,  the  suggestion  that  the  syenite  dikes 
and  the  ores  are  final  products  of  the  same  magma  from  which 
the  gabbro  crystallised  remains  a  rather  attractive  hypothe- 
sis. 

'•  s=*  i  tfe 
The  syenite  contains  numerous  miarolitic  cavities,  which 

may  be  regarded  as  a  strong  indication  that  gases  played  an 
important  r3le  both  in  the  formation  of  the  differentiate  and 
in  the  determination  of  the  character  of  the  crystallization. 
The  abundance  of  hornblende  in  the  gabbro  near  the  edges  of 
the  dike  also  testifies  to  the  importance  of  mineralizers. 
The  concentration  and  formation  of  the  various  ore-bodies  was 
very  probably  the  work  of  the  same  agencies. 

The  ores  are  magmatic  in  the  sense  that  they  are  associa- 
ted in  close  relationship  with  the  feldspar  of  the  syenite 
dikes.  The  ore-minerals,  however,  are  later  than  the  rook 
minerals,  and  were  probably  formed  either  during  the  closing 
phases  of  the  magmatic  period  or  under  pneumatolytic  condi- 
tions. The  abnormal  character  of  the  dikes  (the  variable  tex- 
ture, the  coarse  grain  usually  with  the  ores,  and  the  abundant 
miarolitic  cavities)  indicates  that  the  concentration  both  of 
the  syenite  and  the  ore  was  a  pneumatolytic  process.  Conse- 
quently it  seems  nearer  the  truth  to  consider  the  ores  to  be 
of  pneumatolytic  rather  than  of  direct  magmatic  origin.  The 


55. 


Diagram  of  Mineral  Sequence 


Minerals 


Magmatic 
Period 


Pneurcato- 
lytic  Pe- 
riod 


Hydro- 

thermal 

Period 


Period 
of  Ox- 
idation 


Lab r ado rite 
(in  gabbro) 

Orthoclase 
(in  syenite) 

Augite 

(in  gabbro) 

Albite 

Muscovite 

Biotite 

Apatite 

Titanite 

Hornblende 

Garnet 

Epidote 

Sericite 

Chlorite 

Chalcopyrite 

Bornite 

Galena 

Tetrahedrite 

Calcite 

Chalcopyrite 

Chalcocite 

Covellite 

Malachite 


. 


J 


at  . 

srti    - 
eJiic 


93  i 


e. 


' 


56, 


almost  complete  absence  in  the  syenite  of  the  usual  alteration 
products  which,  commonly  accompany  ores  of  hydrother-nal  origin 

indicates  that  the  sulphides  in  the  dikes  were  probably  formed 
under  earlier  and  probably  pneumatolytic  conditions. 

The  period  of  ore  formation  in  the  wall-rock,  however,  con- 
tinued through  the  transition  from  gaseous  to  hydrothermal  con- 
ditions, as  is  shown  by  the  association  of  epidote  and  chlorite 
with  the  sulphide  stringers  in  the  gabbro. 

SUMMAP.Y 

The  chalcopyrite-bornite  ores  of  La  Fleur  Mt»  occur  in 
unaltered  syenite  and  syenite-pegmatite  dikes,  and  to  a  slight 
extent  associated  with  minerals  of  hydrothermal  origin  in  ad- 
joining gabbro.  The  syenite  and  syenite-pegmatite  may  have 
been  late  extracts  fron  the  magma  from  which  the  gabbro  crys- 
tallized, but  there  is  no  positive  evidence  to  establish  this 
point.  The  formation  of  the  syenitio  magma  and  the  concen- 
tration of  the  sulphides  are  believed  to  have  been  caused 
largely  by  pneumatolytic  agencies.  The  sulphides  are  later 
than  the  rock  minerals  of  the  dike,  but  were  not  accompanied 
by  changes  of  hydrothermal  nature.  In  the  gabbro,  the  occur- 
rence of  epidote,  chlorite  and  sericite,  associated  with  the 
sulphides,  indicates  that  the  mineralization  in  the  wall  rock 
took  place  under  milder  conditions  than  in  the  dikes. 

The  alteration  of  the  ores  by  surface  agencies  is  slight, 


57, 


but  affords  a  clear  oxanplc  of  bornite  altering  to  chaloo- 
cite  and  covellite  with  the  development  of  secondary  chal- 
copyrite  in  fine  lattice  structures  in  parts  of  the  bornite 
protected  fro.-n  direct  attack  by  the  oxidizing  agencies. 


58, 


The  Evergreen  Mine.  Gilpin  Co..  Colorado. 

INTRODUCTION 

The  copper  ores  of  the  Evergreen  Mine  have 
attracted  the  attention  of  geologists  during  the  last  few 
years  on  account  of  the  intimate  association  shown  to  exist 
there  between  the  sulphides  of  copper  and  the  apophyses  of 
a  neighboring  stock  of  monzonite.  The  copper  minerals, 
chiefly  bornite  and  chaloopyrite,  occur  largely  in  two 
small  igneous  dikes,  and  in  the  matrix  of  an  igneous 
breccia  formed  by  the  complex  intrusion  of  the  molten  mass 
into  the  walls  of  shattered  schist  and  pegmatite. 

Situation*  The  little  town  of  Apex,  near  which 
the  Evergreen  Mine  is  situated,  is  well  in  the  heart  of 
the  Front  Range  of  the  Rooky  Mountains,  in  the  high  region 
seven  miles  east  of  the  Continental  Divide.  The  nearest 
railway  point  is  Central  City,  about  6  miles  to  the  south- 
east, which  is  the  terminus  of  a  narrow  gauge  line  of  the 
Colorado  and  Southern  Railway,  which  ascends  the  rugged 
canons  of  Clear  Creek  and  its  northern  tributary  from  the 
town  of  Golden.  The  topography  is  mapped  on  the  Central 
City  sheet  of  the  U.S.C.S. 

Development.  The  mine  has  been  operated  in  a 
small  way  for  nine  or  ten  years.  A  shaft  about  350  feet 
deep  has  been  sunk,  and  levels  run  r.t  100',  200'  and  350' 


59, 


respectively.   A  little  ore  has  also  been  mined  from  a 
tunnel  penetrating  the  deposit  from  a  point  on  the  surface 
north  of  the  shaft.  A  email  mill  has  been  erected  recently, 
and  at  the  time  of  my  visit  some  experimental  work  with 
flotation  methods  was  being  done.  Very  little  ore,  however, 
has  been  shipped. 

Literature.  In  the  first  published  description 
of  the  deposit,  a  paper  by  E.  A.  Hitter1,  the  opinion  was 
expressed  that  the  ores  are  primary  constituents  of  the 
intrusive  rock,  and  crystallized  directly  from  the  magm^. 
with  the  rook  minerals*  The  unusual  concentration  of  the 
sulphides  is  regarded  as  a  segregation  caused  by  sublimations 
from  the  magma*  The  ore-bearing  intrusive  is  described  as 
a  rook  composed  "of  quarts,  alkali  feldspars,  orthoolass 
and  albite,  (often  interlocked  as  mioroperthitio),  with 
augite  of  the  aegirine  variety  and  long  needles  of  enstatite 
and  diallage",  and  the  name  Evergreenite  is  given  it*  Phases 
termed  miorogranitio  and  porphyritic  are  mentioned,  the 
latter  being  regarded  as  a  more  advanced  stage  of  the  segre- 
gation as  it  is  characterized  by  a  greater  abundance  of 
enstatite  and  diallage,  or  of  quarts.  The  mention  of  micro- 
pegmatite  in  the  "microgranitic"  phase  is  of  interest,  and 
the  structure  ie  well  shown  in  a  photograph  of  a  thin  section. 
Two  types  of  alteration  of  the  country  rock  (a  biotite 


1.  E.  A.  Rittsr,  The  'Evergreen  Copper  Deposit,  Colorado, 
T.A.I.E.E.,  Vol.  38;  pp.         >,(1907). 


60, 


schist)  were  observed  by  Mr.  Hitter,  viz., 

(1)  an  introduction  of  quartz  and  aegirite 
forming  pseudo-schists,  and 

(3)  the  formation  of  pseudo-gneisses  by  the 
development  of  alkali  feldspars  and  aegi- 
rite, the  biotite  remaining  little  al- 
tered* 

Covellite  la  described  as  an  alteration  of  bornite  and  ohalco- 
pyrite  due  to  descending  waters,  but  it  is  not  common,  and  is 
believed  to  be  confined  to  the  ore  adjacent  to  the  schist  and 
not  in  the  massive  intrusive. 

Baetin  and  Hill  in  a  later  article1  add  new 
data  concerning  the  mineralogy  of  the  ore,  and  come  to 
some  interesting  conclusions  regarding  Its  genesis*  The 
colorless  pyroxene  in  the  Intrusive  is  shown  to  be  wollaaton- 
ite,  and  not  enstatlte  or  dlallage  as  determined  by  Hitter. 
The  occurrence  of  garnet  in  the  rook  with  the  wollaston- 
ite  is  regarded  as  suggestive  of  the  digestion  of  oal- 
oareous  wall  rook  by  the  magma,  probably  before  it  reached 
its  present  position.  The  authors  support  Hitter's  view 
that  the  sulphides  are  primary  constituents  of  the  dike 
rook,  but  th~y  believe  them  to  have  be^n  derived  either 
from  the  wal}  rocks  at  depth  by  absorption  or  from  the  m&gma 
by  differentiation.  Their  conclusion  Is:—  "The  deposit 
under  this  view  represents  an  endomorphic  effect  produced 
by  contact  metamorphism  at  the  border  of  a  large  intrusion 
of  monzonite." 

1.  E.  8.  Bastin  and  J.  M.  Hill.  The  Evergreen  Cppper 
Mine.  Colorado.  Econ.  Geol.  Vol  6."  p.  465-4    (1911). 


61, 


Physiographic  Features.  The  Evergreen  Mine  is 
at  an  altitude  of  9800,  in  a  region  of  rather  subdued  re- 
lief, slightly  above  the  line  of  the  preoipitouo  elopes 
of  the  deep  canons.   If  the  great  relief  due  to  the  youth- 
ful V-shaped  gorges  of  the  present  roaster  streams,  is  ig- 
nored, the  topography  above  the  8000  foot  contour  is  rather 
subdued;  the  a lopes  are  gentle,  the  ridges  well  rounded  and 
in  places  flat-topped,  and  the  valleys  open  and  with  fre- 
quent meadows.  The  highest  peaks  of  the  range  rise  3000 
or  4000  feet  above  this  general  surface,  but  with  surpris- 
ingly gentle  and  regular  elopes  for  such  high  altitude's. 
Glacial  cirques  interrupt  the  smoothness  of  their  contours 
for  the  last  thousand  feet  or  more,  but  the  glacial  erosion 
is  far  from  mature,  and  has  failed  to  destroy  the  older 
surfaces  completely.  From  the  neighborhood  of  the  mine, 
I  saw  few  peaks  which  could  not  be  ascended  on  horseback. 
The  high  surface  of  relatively  gentle  relief  ie  sharply 
incised  by  the  master  streams  of  the  region,  and  the  cutting 
has  worked  back  alonjr  the  tributaries  in  varying  degrees. 
Between  Black  Hawk  and  Golden,  the  North  Fork  and  main  stream 
of  Clear  Creek  have  cut  a  canon  SOOO  feet  deep,  in  places 
with  high  cliffs,  and  many  of  the  smaller  valleys  of  the 
same  ^nage  basin  present  strikingly  abrupt  slopes,  as  is 
shown  in/figures.  The  three  distinct  topographic  features, 
which  have  been  described  above,  may  be  summarised  as 
follows:  — 


61A 


PLATE  I. 
The  Svergr een  Mine.  Gilpin  Co..  Coloradp 

Fig.  1.  The  Evergreen  Mine  from  the  aouth  east. 


Fig.  ?..  The  Evergreen  Mine  and  the  valley  of  Pine  Creek 
from  the  eouth. 


613, 


PLATE  II. 
The  Region  near   the  Ever  green  Mine.  Gilpin  Co..   Colorado. 


Fit?     *       The  valley  of  Clear  Creek   looking  south  east  frorr, 
a  point  212  miles  northwest  of  Central  City. 


Fig.  The  valley  of  Pine  Creek,   looking  south   from  a 

point  near  the  Evergreen  Mine. 


62, 


(1)  the  old  surface  of  moderate  relief, 

(£)  the  sharp  V-shaped  valleys  dissecting  it,  and 

(3)  the  glacial  cirques  modifying  the  slopes 
of  the  higher  peaks.1 

The  Evergreen  ore-body  io  in  a  shallow  valley 
of  the  older  cycle  of  erosion  in  the  region  of  moderate 
topographic  relief.  It  ia  still  above  the  reach  of  the 
actively  cutting  youthful  streams.  The  climate  is  severe, 
the  snow  las  tin-;  for  seven  months,  but  the  under-ground 
workings  are  rarely  below  freezing. 

General  Geologic  Features.  The  district  is  in 
the  wide  area  of  pre-Canbrian  rocks,  which  form  the  greater 
part  of  the  Front  Range,  the  oldest  formation  ie  a  fine- 
grained biotite  schist,  believed  to  be  of  sedimentary  ori- 
gin.  Into  it  are  intruded  several  plutonic  rocks  also  of 
pre-Cambrian  age,  showing  various  degrees  of  metamorphism. 
Pegmatitio  intrusions  associated  with  the  granites  are  very 
prominent,  in  places  cleaving  the  fissile  schists  as  broad 
dike e  tens  of  feet  In  width,  elsewhere  forming  numerous 
fine  laminae  in  the  earlier  rock. 

The  only  rocks  younger  than  the  pre-Cambrian  are 
various  early  Tertiary  intrusivee,  chiefly  monzonitic  in 
character,  which  form  many  small  stocks  and  dikes.  The 
longer  axis  of  these  later  bodies  is  usually  roughly  par- 
allel to  the  schistosity  of  the  metamorphic  rocks,  although 
in  some  cases  the  intrusions  are  quite  irregular.  The 


1.  Spear  and  Carrey  also  recognize  similar  topographic 
features  in  the  Georgetown  Quadrangle.   (Prof.  Paper  63,  U.S. 
G.S.,  The  Geol.  of  the  Georgetovm  Quadrangle,  Colorado.) 


63. 


chief  ore-bodies  of  the  region,   the  gold-silver  veins  and 
the  tungsten  veins,   are  clearly  related  to  the  Tertiary 
intrusivee,   and  no  claim  in  thoroughly  satisfactory  to  the 
prospector  which  does  not  show  an  outcrop  of  porphyry. 

DESCRIPTION  OF   THE   DEPOSIT 

The  occurrence  of  copper  in  workable  amounts  la 
rare   in  the  district  and  the  ore-body  on  the  Evergreen  Claim 
is  the  only  one  which  supports  a  mine.     The  dependence  of 
the  ore  on  the  dike  rocke  is  unusually  definitely  shown, 
and  the  evidence  read  from  the  relations  there,  greatly 
strengthens  the  general  belief  that  the  "porphyries"  were 
the  mineralizing  agents  throughout  the  region* 

Associated  Rocks.     The  wall  rock  of  the  deposit 
is  the  pre-Oambrian  biotlte  schist,  with  numerous  tabular 

v  lenticular   intrusions   of  a  siliceous  pegmatite.     Although 
nor.  the  thin  sections  show  the  schist  without  some  altera- 

tion due  to  the  mineralizing  fluids,   the  original  rock  may 
be  readily  inferred  to  be  composed  of  fine  flakes  of  a 
deep  brown-green  biotlte  (deep  brownish-green  parallel  to   c; 
pale  green  perpendicular  to   c) ,  and  a  fine-grained  aggregate 
Of  orthoclase  and  quarts.     The  pe;i«atitic  material  is 
chiefly  quartz  and  orthoclase,   in  coarse   irregular  masses* 

To  the  east  of  the  mine,   a  stock  of  monzonite 
outcrops,   and  the  two  dik-:s  with  which  the  ore   is  associated, 
are  very  probably  apcphyees  of  this   stock.     The  dikes  strike 


64. 


0 

a  little  west  of  north,  and  dip  steeply  (60-70  )  to  the 


east,  Leinc  approximately  parallel  both  in  strike  and  dip 
to  the  structure  of  the  schist*  The  dikes  are  about  twenty 
feet  apart.  The  one  to  the  eaat  is  the  larger,  averaging 
13-15  feet  thick,  while  the  one  to  the  west  varies  from  a 
few  inches  to  about  three  feet* 

In  places  the  wall  rocks  are  penetrated  irregu- 
larly by  the  intrusion  of  the  igneous  material,  producing 
masses  of  igneous  breccia  often  of  notable  extent*  The 
rooks  between  the  two  dikes  have  suffered  this  breociation 
particularly.  Places  were  observed  in  which  the  identity 
of  tha  dikes  themselves  becomes  loet  in  a  confused  mass  of 
this  shattered  schist*  All  gradations  exist  from  the  ratio 
of  wall  rook  to  intrusive  in  which  the  schist  fragments  are 
properly  described  as  inclusions,  to  the  conditions  in  which 
the  wall  rock  predominates,  and  the  igneous  rook  fills  ir- 
regular fractures  in  it.  The  schist  and  pegmatite  appear 
as  if  shattered  by  violent  explosions,  and  then  oemented 
by  tha  intrusive  rock.  The  fragments  of  the  breccia  vary 
from  small  chips,  a  fraction  of  an  inch  in  thickness  to 
blocks  a  couple  of  feet  wide* 

The  normal  dike  rock  is  a  fine-grained  hypidi- 
omorphic  granular  rock,  of  a  medium  gray  color,  with  small 
feldspars  and  pyroxenes  visible  with  the  hand-lens.  For 
the  most  part  it  is  even-grained,  but  in  places  there  is 
a  sub-porphyritic  development  of  the  orthoclase.  Dnder 


65. 


the  microscope,  the  unmineralized  rook  is  seen  to  contain 
orthoclase  as  the  chief  constituent  (in  part  mioroperthitic 
with  alblte)  with  somewhat  less  abundant  quarts.  A  small 
amount  of  oligoolase  is  sometimes  present.  Green  augite 
(non-pleocroic  or  only  faintly  pleocroic)  is  the  most 
abundant  dark  mineral,  but  a  little  pale  blue-green  amp- 
hibole  was  observed  in  one  elide,  and  a  few  laths  of  deep 
green  biotite,  (deep  green  parallel  to  c,  light  yell. 
green  perpendicular  to  c).  Titanite  crystals  and  small 
prisms  of  apatite  and  zircon  are  common  accessories* 
The  orthoolase  grains  average  about  .5-  .75  mm.  in  longest 
diameter,  with  a  maximum  of  abovt  1..5  mm;  the  quartz 
usually  about  .5  mm,  and  the  pyroxene  .5   m,  with  a  maxi- 
mum of  1  mm.   'rom  the  Microscopic  work,  the  rook  seems 
more  nearly  a  granite  in  mineral  composition  than  a  mon- 
aonite. 

Approaching  the  ore-bodies,  the  dik«  rook  alters 
to  a  peculiar  alkalins  granite,  with  aegirite-augite,  and 
fine  laths  of  wollastonlte  usually  in  great  abundance. 
In  the  hand-specimen,  it  ie  a  llrht  colored,  fine-grained 
rock  with  a  peculiar  pearly  luster,  distinctly  different 
from  a  normal  feldapathio  rock.   In  some  cases,  the  l&ths  of 
wollaetonita  are  sub-parallel;  in  others  they  form  an 
int-rlaoing  mass,  orthoclaae  with  some  miorooline,  quartz, 
wollaetonite  and  the  deep  green  aegirite-aupjite  are  the  chiff 
components  of  the  rock.  A  little  nlagioclase  (oli^oolase  - 


66 


alblte?),  good  sphenes  of  titanits,  and  email  crystals  of 
airoon  are  accessory. .  Apatite  is  abundant,  in  some  cases 
as  large  grains.  The  aesirlte-augita  forms  small  stubby 
crystals,  usually  with  a  pale  green,  non-pleocroic  core 
which  is  in  striking  contrast  to  the  deep  green  tints  of 
the  outer  portions  of  the  grains.  The  extinction  angles 
of  both  portions  of  the  crystal  are  high,  but  di ffer  by 
a  few  degrees,  the  outer  portion  having  a  slightly  lower 
angle  of  extinction  than  the  inner.  The  feldspars  and 
quartz  are  In  the  usual  hypidiomorphio  relations,  and  of 
about  the  same  grain  as  described  for  the  normal  rook. 
The  wollastonite,  which  is  the  most  unusual  feature,  is 
In  long  laths,  (max.  about  •  75  mm.),  commonly  in  sub- 
parallel  orientation.  It  is  most  ordinarily  grouped  along 
the  contacts  of  grains,  and  tends  to  avoid  the  quarts, 
although  a  few  prisms  penetrating  the  latter  were  observed* 
It  projects  into  the  feldspars  in  some  caeea,  but  the  re- 
lations may  be  explained  equally  well  by  assuming  a  re- 
placement ori  in  for  it,  as  by  assuming  it  to  be  a  primary 
rock  mineral.  The  parallel  structure  shown  by  the  wollaston- 
ite  is  not  shown  by  the  other  rock  minerals  except  to  a 
slight  degree. 

Bastln  and  Hill  mention  finding  abund- 
ant garnet  in  the  dike-rock,  and  in  the  breccia,  but  in 
the  thin  sections  of  my  specimens  it  is  not  an  important 
mineral.  The  unusual  abundance  of  apatite  does  not  seem 


• 

• 


• 


• 

• 


67, 


to  have  been  emphasised  by  the  previous  writers. 

The  dike  rook  containing  the  sulphides,  and  the 
rook  forming  the  iprneous  breccia  is  the  aegirite-augit* 
and  wollastonite  bearing  phase  of  the  granite.   For  the 
most  part,  the  thin  western  dike  (the  main  Evergreen  dike) 
consists  entirely  of  the  vvollastonlte  bearing  rook,  but  on 
the  300*  level,  where  a  drift  has  been  pushed  several  hun- 
dred feet  to  the  northwest  along  it,  it  changes  without  any 
distinct  boundary  into  the  normal  gray  granite.  In  one 
place  the  change  required  about  10  feet,  -  in  another  it 
was  apparent  within  the  dimensions  of  a  hand-specimen. 

The  wollastonite- bear ing  rock  la  probably  the 
rock  named  "Evercr*enite"  by  Hitter,  but  as  it  has  been 
proved  by  later  work*,  to  be  of  different  mineralogioal  com- 
position and  as  it  is  merely  a  Modification  of  an  alkaline 
granite,  it  seems  hardly  worth  while  to  burden  the  already 
bewildering  list  of  rock  types  with  another  name.  The 
name,  however,  eervee  the  local  needs  admirably,  and  will  be 
used  to  refer  to  the  wollaetonite  bearing  phase  of  the  alka- 
lian  granite  associated  with  the  ores. 

Primary  Mineralization  and  Rook  Alteration.  The 
common  ore-minerals  are  ohaloopyrite  and  bornite,  present  in 
about  equal  amounts.  The  largset  masses  of  the  two  sulphides 
occur  in  the  "evergreen it e»,  usually  not  in  the  dikes  th-em- 


1.  Baatin  and  Hill,  Loo.  oit. 


68. 


selves  but  in  the  irregular  intrusions  in  the  breooiated 
wall  rook.  The  inclusions  of  schist  or  r»g«atite  contain 
fine  seams  of  ohaloopyrlte,  or  disseminated  specks  of  ohaico- 
pyrite  or  bornite,  but  no  large  masses  of  the  ore-minerals 
were  observed  except  in  the  igneous  matrix  itself.  Thd 
dike  rock  associated  with  the  ore  contains  numerous  mlaro- 
litic  cavities,  and  in  places  develops  a  coarser  structure, 
suggesting  pegmatitic  tendencies.  The  presence  of  micro- 
pegmatite  in  these  rocks  mentioned  by  Ritter  is  in  accord 
with  this  observation.  In  the  miarolitic  cavities  crystals 
of  orthoolase,  with  crocidolite,  and  imperfectly  crystallized 
grains  of  bornite  and  chaloopyrlte  between  them,  were  ob- 
served in  a  few  cases.  Fluorlte  occurs  in  small  grains  in 
the  intrusive,  usually  in  the  miarolitic  portions.  A  few 
particles  believed  to  be  tourmaline  were  observed,  but  were 
not  positively  identified. 

Hydrothermal  minerals  are  not  common,  but  epidote 
is  associated  with  the  bornite  in  many  oases.  It  fringes 
the  bornite  grains,  replaces  the  feldspars  and  pyroxenes, 
or  cuts  across  the  rook  in  veinlete. 

Serioite  is  somewhat  developed  in  the  feldspars, 
but  on  the  whole  it  is  not  very  abundant.   In  one  of  two  oases, 
laths  included  in  th?  outer  portions  of  bornite  graina  were 
observed,  and  interpreted  to  indicate  the  extension  of  the 
period  of  bornite  formation  into  the  period  of  conditions 
favorable  for  sericite  (I.e.  hydrothermal). 


69, 


Calcite  ie  common,  replacing  both  the 

earlier  rook  minerals,  and  thf  vollaatonite.  The  latter 
ie  particularly  susceptible  to  the  attack  oi'  the  caloit?, 
and  in  many  specimens,  its  previous  existence  in  the  rock 
can  be  inferred  only  from  the  lath-like  forma  assumed  by 
the  oalcite.   The  caloite  seems  a  little  later  than  the 
epidote,  and  is  moat  probably  a  product  of  the  closing 
phases  01  tn.;  enan&tione  which  formed  the  ores* 

AB      in  the  field  and  in  the  hand-specimens, 
the  achiets  present  fairly  sharp  boundaries  against  the 
invading  dike-rock,  but  in  a  few  places,  intimate  injections 
p-lor.  ,  the  cleav.--5j.es  cf  small  fragments  have  reduced  them 
to  mere  bands  oi  the  dark  constituents  (chiefly  biotite) 
floating  in  coarser  grained  bands  of  the  intrusive.  Under 
the  microscope,  the  schist  is  saan  to  contain  in  many 
cases,  a  blue-green  amphibole  in  broad,  rather  irregular 
grains,  up^to  r5  to  ,75  mm.  in  length,  which  commonly  con- 
tains abundant  inclusions  of  fine  flakes  of  the  biotite  of 
the  schist*   (The  microscopic  properties  fail  to  identify 
the  amphibole  with  any  of  the  ordinary  varieties.  They  may 
be  summarized  as  follows:—  Index  and  birefringence  some- 
what 1&3&  than  augite;  amphibole  cleavage;  oblique  extinc- 
tion to  c,  max*  =ua;jle  41°;  pleocroic,  light  blue-green  para- 
llel to  A;  optio&lly  poeitive(?).  Pale  cores  with  more 
deeply  colored  rims  are  common*  The  change  is  accompanied 
by  a  slight  incree^e  in  the  angle  of  extinction).  The  longer 


70. 


axle  of  the  amphibole  is  usually  parallel  to  the  achistosity. 
Its  similarity  to  the  amphibole  observed  in  the  normal  gran- 
ite, and  the  inclusions  of  biotite  favor  the  view  that  it 
is  a  product  of  emanations  from  the  intrusive,  reacting  with 
the  min-srala  of  the  schist. 

The  borders  of  the  schist  inclusions  may  be  seen 
with  the  aid  of  the  microscope  to  be  usually  lined  with  fine 
grains  of  deep  green  aegirite-augite.  Fine  grains  of  augite 
are  abundant  throughout  the  inclusions,  but  as  they  occur 
along  veinlats  in  a  few  places,  and  seem  closely  related  to 
the  aegirite-augite  near  the  margins,  they  were  probably 
introduced  from  the  magma.  The  augite  grains  do  not  occur 
as  inclusions  in  the  amphibole  in  the  echist,  and  are  prob- 
ably later. 

The  feldepathic  portions  of  the  schist  are  partially 
reorystallized,  especially  near  the  edges  of  the  inclusions. 
Large  grains  of  orthoolaee,  surrounded  by  fine-grained  aggre- 
gates of  biotite  and  augite,  although  united  in  one  fairly 
definite  crystal,  still  show  by  their  wavy  extinction,  the 
irregular  interlocking  boundaries  of  the  grains  from  which 
they  were  welded.   Irregular,  elongated  grains  of  dirty 
looking  titanite  are  common,  frequently  associated  with  a 
peppery  aggregate  of  small  magnetite  specks.  Both  minerals 
are  probably  original  constituents,  but  thay  seem  to  have 
suffered  many  hardships  in  their  severe  experiences  of  suc- 
cessive intrusions. 


71. 


The  pegmatitic  material  la  little  altered  by  the 
dike-rook.  The  occurrence  of  coarse  quartz  in  certain  breo- 
oiated  «ones  is  rather  puzzling  to  interpret,  but  it  is  most 
probably  pre-Cambri?n  material  for  very  siliceous  phases  of 
the  pegmatites  were  observed.  The  ooareer  types  of  the  ore- 
bearing  rock  become  almost  pegmatitio  in  small  areas,  but 
rarely  approach  the  grain  of  the  older  material* 

Epldote  and  sericite  are  developed  to  varying  de- 
grees in  both  types  of  the  wall  rook.  The  fine-grained  feld- 
spars of  the  sohists  are  especially  susceptible  to  the  attack 
of  the  sericite,  but  they  and  the  other  minerals  of  the  schists 
usually  show  less  alteration  to  epidote  than  the  minerals  of 
the  Igneous  matrix. 

The  bornite  and  ohaloopyrite  are  present  in 
approximately  equal  amounts.  The  contacts  between  the  two 
minerals  are  commonly  sharp  and  definite,  with  smooth  un- 
broken lines,  and  as  commonly  showing  blunt  penetrations  of 
the  bornite  by  chaloopyrite,  ae  of  the  ohalcopyrite  by  the 
bornite;  in  brief  the  minerals  exhibit  mutual  boundaries! 
toward  each  other.  In  a  number  of  specimens,  the  graphic 
structure  is  developed,  usually  with  the  chaloopyrite  as 
the  host  mineral,  and  with  bornite  as  the  smoothly  irregular 
blebs  in  it.   (Figures  45,46. )  When  these  structures  are 
studied,  it  is  difficult  to  avoid  forming  the  opinion  that 


1.  Page  51 
S 


the  two  sulphid  so  are  contemporaneous  in  origin.  In  a 
fair  number  01  casee,  however,  bornite  was  observed  to  form 
broad  margins  alorv,  gangue  veinlets,  now  altered  to  calcite, 
which  out  chalcopyrite  areas.   (Figure  42.)  •  The  deduction 
that  thie  bornite  is  later  than  the  chalcopyrite  follows 
inevitably,  and  it  is  -:<leo  clear  that  the  bornite  is  of  the 
same  age  or  later  than  the  original  gangue  mineral  in  the 
vein.  No  break  can  be  detected  between  the  bornite  of  the 
areas  showing  the  mutual  boundaries  or  the  graphic  structure, 
and  the  bornite  of  the  ve inlets.  The  most  acceptable  hypothe- 
sis is  that  the  chief  masses  of  the  ohalcopyrite  and  bornitt 
are  of  contemporaneous  origin,  but  that  the  bornite  contin- 
ued to  form  after  chaloopyrite  deposition  had  ceased*  The 
question  may  arise  if  there  is  any  possibility  that  the  bor- 
nite which  replaces  the  chalcopyrite,  is  of  secondary  origin, 
i.e.  deposited  from  cold  descending  surface  waters.  In  a 
deposit  exposed  only  to  a  depth  of  200  feet,  this  possibility 
cannot  be  absolutely  eliminated,  but  the  slight  amount  of 
secondary  chalcocite,  the  lack  of  any  epacial  relations  be- 
tween the  bornite  veins  and  the  chalcocite,  the  association 
of  the  bornite  with  hydrothermal  minerals,  and  tiuv  rarity  of 
bornite  as  a  secondary  mineral  elsewhere  argue  convincingly 
against  this  view* 

The  bornite  is  confined  almost  exclusively  to  the 
dikes  or  to  the  igneoue  matrix  of  the  breccia.  The  chalco- 
pyrite, however,  is  not  uncommon  as  seams  or  finely  dieserci- 


73. 


nateci  grains  throughout  the  eohist  inclusion,.  This  may  be 
considered  to  indicate  that  the  alteration  of  the  schists 
took  place  chiefly  during  the  early  intense  phases  of  the 
intrusion,  and  in  th*  milder  phases  under  which  the  bornite 
became  of  increasing'  importance,  the  alterations  were  confined 
to  the  coarser-grained  dike  rock.  The  intense  alteration  of 
the  min-rals  of  the  schists  to  amphlbole  and  augite,  with  the 
much  slighter  development  of  sericlte  and  epidote  is  parallel 
evidence  supporting  this  interpretation,  but  both  of  these 
sets  of  facts  may  be  due  to  chemical  differences  in  the  rock. 

Tetrahedrite,  galena  and  sphalerite  occur  in  snail 
amounts  with  the  bornite  and  chaloopyrite.  The  tetrahedrlte 
nd  galena  are  probably  contemporaneous  *ith  the  bornite, 
occurin  as  scattered  blebs  or  in  places  abundantly  enough 
to  develop  small  areas  of  the  graphic  structure.  This  is 
especially  noteworthy  with  respect  to  the  galena.   In  a  few 
cases  the  sphalerite  contains  abundant  fine  inclusions  of 
chalcopyrite,  which  suggest  an  intdrgrowth. 

Secondary  products.  Chalcocite  is  not  uncommon 
under  the  microscope,  but  it  is  rarely  abundant  enough  to  be 
visible  in  the  hand-specimen.  It  occurs  sparingly  as  blebs 
in  the  bornite,  the  two  sulphides  exhibiting  mutual  boundaries, 
but  generally  it  is  in  veinlets  or  in  rime  about  the  bornite, 
or  less  commonly  about  the  chalcopyrite.   In  these  forme  it 
is  clearly  of  replacement  origin. 

The  ve inlets  of  chalcocite  penetrating  the  bornite 


• 

•• 


74. 


are  frequently  oriented  parallel  to  several  orystallo^raphic 
directions  in  the  bornite,  of  which  two,  three  or  rarely 
four  are  revealed  on  the  polished  surface,  and  form  a 
pattern  which  we  have  termed  the  lattice  structure.  The 
structure  will  be  described  in  greater  detail  for  other 
deposits  in  which  it  is  a  more  prominent  feature. 

Associated  with  the  chalcccite,  but  more  abundantly 
developed,  is  chalcopyrite  of  a  second  generation,  which 
commonly  forms  plates  penetrating  the  bornite  from  cracks 
or  grain  boundaries*  On  the  polished  surface,  the  plates 
appear  as  strips  or  spines  of  chalcopyrite,  usually  oriented 
in  two  or  three  directions,  yielding  lattice  structures  simi- 
lar to  those  mentioned  above*  The  later  chalcopyrite  can  be 
easily  distinguished  from  the  earlier.  The  replacement  of 
bornite  by  chalcopyrite  of  this  second  generation  is  never 
complete,  and  in  all  oases  the  outline  of  the  original  grain 
is  sharply  defined  even  when  againet  th?-  earlier  chelcopyrite 
(figures  43,45.)  The  two  agea  of  chalcopyrite  are  very  clearly 
shown  by  the  replacement  of  the  earlier  ohalcopyrite  by  bornite 
along  certain  gangue  veinl^ts,  and  by  the  partial  replacement 
of  the  bornite  in  turn  by  spines  of  chalcopyrite  developed 
later  alon^  the  margins  of  the  same  gangue  veinlets. 
(Figure  42.)  .  The  development  of  chalcopyrite  of  the  second 
generation  is  accompanied  by  the  formation  of  numerous  email 


1.  Engels^.lSS;  Seven  Devils, p. 147;  Bisbee,p.l74;  Kennecott, 
pp. 358, 261. 

(u 


75. 


cracks,  which  suggest  that  the  reaction  involves  a  shrinkage 
in  volume.  This  feature  places  the  chalcopyrite  of  the  two 
ages  in  striking  contrast.   (Figure  45.  ).  The  development 
of  the  later  ohalcopyrite  slightly  preceded  the  formation  of 
the  chalcocite,  but  the  general  distribution  of  the  chalco- 
pyrite and  its  dependence  on  the  same  ohannel-waye  as  the 
chalooeite  afford  clear  evidence  that  the  two  minerals  were 
produced  by  the  same  agencies. 

In  one  specimen,  the  chalcocite  areas  are  broken  by 
a  feathery  group  of  small  veinlets,  alon^;  or  related  to  which 
are  oxidised  products  and  small  plates,  or  blebs  of  ohalco- 
pyrite and  bornite.   (Figure  41.).   In  a  rough  way,  the 
structure  is  parallel  to  the  bornite-chalcocrite  lattice  pat- 
terns in  the  same  chalcocite  areas,  but  the  bornite  blebs 
usually  do  not  resemble  residues.  The  intervening  chalooeite 
is  clean  and  free  from  inclusions  of  any  sort.  The  relations 
suggest  the  possibility  that  the  usual  reactions  of  enrich- 
ment may  have  been  locally  reversed  by  a  concentration  of  out- 
ward migrating  iron,  and  that  this  bornite  and  chalcopyrite 
in  small  amount  was  formed  by  the  replacement  of  chalcccite. 

The  tetrahedrite  is  fairly  resistant  to  the  attack 
of  the  second  generation  of  chalcopyrite.  Spines  of  the  latter 
in  bornite  were  observed  to  end  abruptly  against  the  sharp 
contact  of  a  tetrahedrite  grain. 

The  relations  in  on^  polished  chip  suggest  that 
galena  alters  more  readily  than  bornite  to  secondary  chaloo- 


net 


76, 


pyrite,  but  the  evidence  is  not  entirely  convincing. 

There  la  a  little  covellite  with  the  chalcocite 
an-t  later  ohalcopyrite,  but  it  ia  almost  negligible  in  amount. 

The  surface  of  tha  ground-water  is  about  33  feet 
below  the  collar  of  the  shaft.  Stains  of  limonite  snd  mala- 
ohite  are  common  along  seams  and  cracks  above  it,  although  the 
amount  of  thoroughly  oxidized  capping  ia  very  small,  end 
rarely  exceeds  a  few  feet  in  thickness. 

Summary  and  Discussion. 

The  minerals  observed  in  the  rooks  and  ores  of  the 
Evergreen  Mine  are  listed  on  the  following  page,  and  their 
sequence ,  based  on  the  observations  presented  on  preceding 
pages,  is  indicated  by  the  lines  of  the  diagram. 

It  should  be  noted  that  the  interpretation  of  the 
diagram  ia  a  little  different  in  the  case  of  the  oxidized  miner- 
als, and  secondary  sulphides.  All  of  these  minerals  are  prob- 
ably forming  at  the  present  time  in  different  parts  of  the  de- 
posit, but  the  order  in  which  they  develop  at  any  one  point  in 
the  ore»_h»w«4E«-g-,  ia  shown  by  the  lines  of  the  diagram. 


77 


Diagram  of  Mineral  Sequence 


Minerals 

Magruatio 
Period 

'neuaatolytic 
Period 

Hydrothertral 
Period 

Period  of 
Oxidation 

1  .   Aucri  te 

•» 

2.    Zircon 
3.   Biotite 
40rtliool£iBfi 



5.   Mi  or  oo  lino 

6Pla.flrloala.so 

70113^112 

SAoati  te 

9.   Titanite 

10.  Wollastonite 

1  1      Arnnliibolft 

As&Br  i  te^ans1!  te 

13.   Crooidolite 
14.   Fluor  ite 
15.   Tourmaline    (?) 
18.    Chalaocvr  i  te 

^>  

:  —  —  — 

17.   Bornit* 

_—  —  • 

—  -  —  _ 

18.  Sphalerite 
19.  Tetrahsdrite 
20  .   Galena 
21.  Epidote 

_ 

22      Sarioita 

23.   Chlorite 

24.    Calaite 

25.   Chaloooita 
26.   Covellite 
27.   Malachite 
28.   Azurite 
29.    Lirconite 
30.   Kaolin 

^—^— 



78. 


The  minerals  grouped  as  magmatio  are  those  which 
are  believed  to  have  crystallized  directly  from  the  igneous 
melt.  Those  termed  pneumatolytic,  are  believed  to  have  been 
formed  at  the  period  in  which  the  concentration  of  gases  was 
great,  and  *hen  conditions  of  equilibrium  were  governed  largely 
by  the  presence  of  mineralizers.  No  sharp  boundary  between 
the  magraatio  period  and  the  pneumatolytic  period  oan  be  drawn, 
as  probably  conditions  favoring  simple  crystallization  from 
the  melt  grade  into  the  more  complex  conditions  of  pneu- 
matolytic alteration  with  the  increasing  concentration  of 
gases  and  their  accompanying  elements.  The  final  and  complete 
consolidation  of  the  rook  probably  took  place  during  the 
pneumatolytic  period.  With  constantly  decreasing  temperatures, 
pneumatolytic  conditions  gave  way  to  those  predominently 
hydrothermal  in  character,  which  mark  the  closing  phases  of 
the  mineralization  connected  with  the  intrusion.  The  whole 
presents  an  unbroken  sequence  from  the  initial  magmatio  con- 
ditions to  the  final  feeble  hydrothermal  effects.  Periods  in 
which  one  or  another  set  of  conditions  dominates  may  be  recog- 
nized, and  their  products  described,  but  definite  breaks  be- 
tween them  do  not  exist. 

The  minerals  placed  in  the  magmatic  group  require 
little  discussion.  Their  sequence  is  that  commonly  observed 
in  igneous  rooks.  Both  the  feldspar  and  quartz,  however,  con- 
tinued to  form  under  the  early  etag'es  of  pneumatolytio  con- 
ditions, if  the  occurrence  of  micropegraatite  noted  by  Hit- 


79 


may  be  ao  interpreted.  The  large  crystals  of  ortho- 
olaee  in  miarolltio  cavities  *ith  the  sulphides  and  cro- 
cidolite  are  further  indication  of  the  formation  of  the 
feldspar  under  the  later  conditions. 

The  aegerite-augite  occurs  as  rims  around  cores 
of  audits,  and  is  believed  to  be  the  result  of  the  shifting 
conditions  brought  about  by  the  increasing  concentration  of 
volatile  constituents*  The  amphibole,  which  is  developed 
chiefly  in  the  inclusions  and  wall  rock  is  probably  also  a 
product  of  pneumatolytic  action.   Its  formation  is  known  to 
be  favored  by  the  presence  of  hydrous  vapors.  The  orocldollte  , 
which  ia  meet  commonly  formed  in  cavities  of  pegmatite  dikes 
may  be  placed  in  this  group  without  question.  The  abundant 
development  or  vratite,  both  in  the  evergreenite  and  in  the 
schist,  is  a  further  indication  of  gas  concentration,  for 
the  exist ance  of  notable  quantities  of  the  mineral  demands  the 
presence  of  ei "her  chlorine  or  fluorine.  Fluor ite,  which  was 
observed  in  small  grains  in  miarolitlc  material,  al*?o  empha- 
sizes the  importance  of  the  pneuaiatolytic  period  in  the 
history  of  the  deposit.  Tourmaline  was  doubtfully  identified 
and  if  present  it  belongs  in  this  group. 

As  has  been  discussed  previously,  there  is  reason 
to  believe  that  the  wollastonlte  is  later  than  the  feldspar 
or  the  quartz  of  the  evergreenite.  Its  rarity  as  a  product 
of  direct  magmatic  crystallization  argues  against  grouping 

1.  Loc.  cit. 


80 


it  with  the  earlier  minerals.   It  is  chiefly  found  as  a 
contact-met amorphic  mineral  in  limestones,  where  the  abund- 
ance of  Ca  0  is  undoubtedly  a  controlling  chemical  factor, 
but  where  at  the  same  time,  the  physical  conditions  of  forma- 
tion are  controlled  by  the  heated  emanatione  from  the  in- 
trusion, and  are  comparable  to  the  pneumatolytic  conditions 
postulated  in  the  genesis  of  the  Evergreen  rocks  and  ores. 
For  these  reasons,  it  seems  better  to  place  the  mineral  in 
the  pneumatolytic  group  in  this  case,  although  the  micro- 
scopic evidence  is  not  absolutely  conclusive  that  it  belongs 
there •  Baatin  and  Kill  describe  the  ore-body  as  an  endo-. 
morphic  effect  due  to  contact  raetamorphism  as  quoted  on  page 
60  •  The  interpretation  of  the  wollastonite  as  a  pneuma- 
tolytic product  is  in  agreement  with  their  conclusion,  al- 
though elsewhere  in  their  paper,  the  impression  ia  given  that 
they  regard  the  mineral  as  a  product  of  direct  crystallization 
from  the  melt. 

The  sulphides  are  clearly  later  than  the  feldspar, 
quart a  and  pyroxene,  for  they  unmistakably  corrode  these  rock 
minerals.  The  occurrence  of  the  sulphidec  in  the  rocks  show- 
ing greatest  concentration  of  aegerite-augite,  wollastonite 
and  apatite,  and  in  rocks  with  abundant  miarolitic  cavities, 
points  to  their  formation  under  pneumatolytic  conditions. 
The  microscopic  relations  show  the  sequence  of  ohalcopyrite 
and  bornite  to  be  approximately  as  indicated  by  the  diagram. 
(The  evidence  is  given  on  pages  71-73.)       The  other 


81, 


sulphides  are  hardly  abundant  enough  to  give  conclusive 
evidence  of  their  age  relations,  but  they  are  approximately 
contemporaneous  *ith  the  bornite,  although  probably  somewhat 
icor?  abundant  toward  the  closing  phases  of  its  period  of 
formation.   It  is  of  int?rf.*t  that  the  amount  of  magnetite 

and  pyrite  As  very  small.  The  little  magnetite  observed 

the 
was  originally  present  in  schists.  The  pyrite  occurs  only 

A 

a.a  a  few  scattered  grains,  apparently  in  the  schist,  and 
may  be  the  product  of  other  mineralizing  conditions,  unaeso- 
elated  with  the-  chief  mass  of  th»  ore-body.  Baetin  and  Hill, 
on  what  seems  to  be  good  evidence  believe  another  p-riod  of 
mineralization  has  introduced  galena  and  sphalerite,  but 
neither  the  field  nor  microeoopic  results  of  my  work,  givo 
any  further  information  on  thie  subject.  The  small  amount  of 
sphalerite  and  galena  to  which  reference  has  previously  been 
made,  is  probably  not  what  they  had  in  mind. 

Epidote,  eericite,  chlorite,  and  calcite  fto-faydr^- 
aiiiuiulB  have  been  classified  ee  hydrotheraml  in  agreement 
with  the  general  interpretation  of  the  conditions  under  which 
these  rrinersle  form.  The  cloee  association  of  bornita  and 
epidote  in  several  caees  indicates  the  continuation  of  the 
formation  of  bornite  under  hydrothermal  conditions,  although 
thrre  le  lese  evidence  of  its  association  with  th«  other 
minerals  ol  the  group.   In  the  rich  ores,  the  wollaetonite 
of  the  evergreenite  is  invariably  rpplaceci  by  calcite;  this 
special  relationship  between  the  sulphides  and  caloitee  sug- 


82. 


gests  that   the   latter  IB  probably  a  late  product  of  the 
mineralizing  agencies,  rather  than  a  result   of  descending 
carbonates.     The   ehallowneos   of  unquestioned  surface  altera- 
tion except  along  narrow  Beams  and  veinlete  aleo  favors  this 
view.     The  oalcite  la  later  than  the  aerioite  or  chlorite 
and  may  be  regarded  as  the  closing  product   of  the  mineralising 
processes. 

The  reaiitte  of  oxidation  are  limited  to  &  •mall 
amount  of  secondary  chalcopyrit*  and  veinlete  of  chalcocite, 
and  to  a  few  f«et  of  malachite  and  liEonit«"'-et*.Ined  capping. 

The   Origin  of  th~  Deposit.   Baetin  and  Hill  state 

the 
that  the  dike  of  evergr«<?nlte  la  s.n  offshoot   cf  rconzcnite 

A 

stock  to  the  east  of  the  min?,  and  there  aeemr  to  be  no 
reason  to  doubt- the  correctness  of  their  statement.  If 
this  is  the  case,  the  deposit  presents  an  interesting  se- 
quence of  differentiations  from  monzonitic  aiataria.1  into 
an  alkaline  granite,  and  locally  into  the  peculiar  sulphide 
e.nd  wollastonite  bearing  rock,  evergreenita.  A  progressive 
change  of  this  sort  may  be  readily  interpreted  as  a  reault  of 
pneumatolytio  action.  The  concentration  of  the  alkaline  ele- 
ments in  magmas  has  be^n  attributed  to  gaseous  action  by 
C.  H.  Smyth,  Jr.,1  who  considers  water  to  be  the  most  im- 
portant and  abundant  miner allzer. 


1.  C.  H.  Smyth,  Jr.,  The  Ohem.  Co.:  n.  of  th*  Alk.  Rocks 
and  its  Signif.  as  4c  their  ori  in,  A.  J.  S.,  Vol.  36,  1915, 
p.  33. 


. 


83. 


Arhenius1  states  that  water  above  the  critical  temperature 
would  tend  to  extract  COg,  &,£,  e,nd  the  univalent  ions  such 
aa  01,  Fl,  B,  with  some  of  the  strongly  positive  ions  as 
Na,  E,  and  the  rare-earths .  With  such  a  mechanicffi  there 
let  good  reason  to  believe  that  the  small  residues  of  valuable 
metals  might  become  concentrated,  and  finally  deposited  in 
forma  rich  enou,^h  to  constitute  ores.  The  entire  proceee 
is  similar  to  that  commonly  postulated  for  the  formation  of 
pegmatites.  The  evidence  of  the  importance  of  gaees  — 
euch  as  the  presence  of  aegiri te-auglte  rime,  of  abundant 
apatite,  of  fluorite  and  possibly  tourmaline,  of  micropegma- 
tite  and  th:j  miarolitic  cavities  —  has  been  presented  and 
discussed  on  the  preceding  pages,  and  it  is  sufficient  I 
believe  to  warrent  ti>e  conclusion  that  the  deposit  is  pre- 
dominantly of  pneumatolytic  origin,  although  the  mineralization 
continued  srith  declining  strength  under  hydrothermal  conditions. 

Ths  unusual  occurrence  of  wollastonite,  however,  is 
less  satisfactorily  explained  by  such  an  hypothesis.   Its 
rarity  as  a  const it\ient  of  an  igneous  rock,  or  as  a  pegmatitic 
mineral  suggests  that  thsre  la  probably  some  peculiar  con- 
dition governing  its  formation  in  this  case.  Baetin  and  Hill 
believe  that  the  high  content  of  Ca  0  in  the  magma  is  dua  to 
ths  reaction  of  tha  melt  on  caicareouo  country  roc1*.  As  the 
boundaries  between  the  inolxided  aahiet  fragment*  and 


1.   S.  Arrheniusj  Zur  Physik  der  Vulkaniswusj  Geoi.  Foren> 
Forh.,  22,  p.  247,  1900. 


84. 


ths  intrusive  exclude  the  possibility  of  any  notable  rr^ 
having  been  absorbed  from  the  wall  rocke  at  the  present  sur- 
face,  tli*y  conclude  aft-  ing  the  v-rlour,   possibilities 
that  the  mac^a  probably  absorbed  the  Ca  0  from  calcareous 
rocke  encountered  .-vt  ^;r--  t   r  depths. 

The  explanation  of  the   origin  of  the  depooit  advanced 
by  Bastin  and  Hill   IE   in  no  way  opposed  to  the  conception  of 
a  pn-ivur.atclytic  origin  for  the   oree,   and  may  be  enlarged  with- 
out difficulty  to  strengthen  the  idea.     According  to  a  theory 
advanced  by  R.   A.   Daly1,        alkaline  rocks  are  the  acid  pol? 
of  the  differentiation  or  &  eyntectlc   formed  by  the  reaction 
between  s,  basic  magma  and  a  calcareous  rock.      A  mechanism, 
chemical  and  mechanical  by  *hich  the  introduced  Ca  CCXj  may 
react  with  the  inagiaa  to  fora  and  concentrate  a  differentiate 
Uigh  in  the  alkalies  la  daecribed  by  Daly2,   and  will  not  be 
di-scuased  here  except   thoee  phases   of  it  which  may  be  of 
direct  application* 

The   increased  concentration  cf  Ca  0  is  believed  to 
r---iult   in  the   formation  of  ths   comparatively  denee  li?T.e-irori- 

ii;-'iuE  silicates,  which  tend  to  concentrate  toward  the  lower 
portions  of  the  magoa,   leaving  the  remaining  material  rela- 
tively richer  in  the  alkalies.     An  ejcoecs   of  Ce.  0,  which  might 
conceivably  separate  as  the  simple  silicate,  wollaatonite, 

McGraw-Hill  Co.  . 

1.     R.   A.    Daly,      Igneous  Rocks  and  Their  Origin,     II.    Y.,  1914, 
p.    430. 

'«.      Ibid.,  p.    430. 


(Ca  31  0-^)  would  have  little  tendency  to  settle  away  from 
the  acid  pole,  as  its  speoifio  gravity  of  2.g  (at  normal 
temperature)  la  but  little  greater  than  that  of  a  monr.onitic 
rook.  The  Importance  of  ths  resurgent  CO,  is  emphasised  by 
Daly  as  a  condition  aiding  the  differentiation  of  the  magma, 
and  evidence  la  given  illustrating  the  manner  in  which  the 
alkalian  elements  are  transported  by  this  gas.  Applied  to 
the  conditions  at  the  Evergreen  Mine,  it  is  evident  that  the 
theory  offers  a  common  explanation  for  the  alkalian  character 
of  the  granite  dike,  the  presence  of  wollastonite,  and  the 
pneumatolytic  nature  of  the  deposit.  The  concentration  of 
sulphides  of  copper  and  subordinate  quantities  of  other  metals 
may  be  attributed  to  resurgent  gases,  chiefly  carbon  dioxide 
and  water,  provided  It  Is  assumed  that  caloareoua  rocks  have 
been  encountered  at  depths, 

Origin  of  the  Chalcooite.  Although  th«  amount  of 
oxidation  is  slight,  it  is  entirely  sufficient  to  have  furnished 
the  very  small  quantity  of  copper  necessary  to  have  formed 
the  thin  rims  or  veinlets  of  chaloooite  in  the  bornite.  The 
similarity  between  the  character  of  the  replacement  of  the 
bornite,  and  the  association  between  the  chaloocite  and  the 
spines  of  chalcopyrite  la  very  similar  indeed  to  the  conditions 
found  in  other  ore-bodies  in  connection  with  chalcocite  of 
known  secondary  origin,  end  there  is  no  reason  for  believing  this 
case  to  be  exceptional.  A.  F.  Rogers  ,  in  an  article  on  seoon- 

~TI Secondary  Sulphide  Enrichment  of  Copper  Ores  with  Special 
Reference  to  Microscopic  Study,  liin.  and  Sol.  Press,  Oct.  31» 
191^-,  p.  6S6. 


86, 


dary  sulphide  enrichment,  suggests  an  origin  by  ascending 
solutions  for  the  chalcocite  at  the  Evergreen  Mine,  and 
publishes  a  photograph  to  support  this  view.  It  is  true  that 
there  is  a  small  amount  of  ohalcooite  in  blebs  in  the  bornite, 
which  is  difficult  to  interpret  as  an  ordinary  replacement 

of  the  bornita,  and  which  is  possibly  of  an  earlier  age  than 

\ 

the  chaloooite  forming  the  veinlets  and  rima.   In  one  or  two 
cases  associated  blebs  of  chalcocite  and  tetrahedrite  in  the 
bornite  show  a  faint  tendency  toward  the  graphic  structure. 
If  limited  to  these  rather  indefinite  forms,  which  surely 
constitute  less  than  5  %  of  the  ohaloocite  present,  Rogers' 
suggestion  carries  weight,  and  can  not  be  disregarded.  There 
is  nothing  in  the  evidence  to  eliminate  the  possibility  that 
this  small  portion  of  the  chalcocite  is  primary,  but  it  is 
fairly  certain  that  the  larger  part  of  the  chaloocite  and 
the  associated  ohalcopyrite  which  replaces  the  bornite  la  due 
to  the  action  of  descending  surface  waters.  Bastin,  in  a 
later  article*  sees  no  reason  for  attributing  the  ohalcooite 
to  ascending  solutions,  and  prefers  the  commoner  explanation 
for  it. 

Conclusions. 

The  conclusion  that  the  ore-deposit  is  the  result 
of  the  differentiation  of  apopbyses  of  a  monzonltio  stock, 


1.  The  ore-deposits  of  Gilpin  Co..  Colorado,  EC on.  Geol.. 
Vol.  I0l (T9l377~  P-  876. 


87 


brought  about  chiefly  by  pneumc.tolytio  agencies,  follows  with 
fair  certainty  from  the  evidence.  The  presence  of  wollaston- 
ite,  according  to  Baetin  and  Hill,  suggests  the  reaction  be- 
tween limestone  and  the  magma  at  depth.  This  conception,  en- 
larged by  Daly's  theory  or  the  origin  of  alkallan  rooks,  offers 
an  explanation  for  all  the  observed  facts,  but  it  rests  upon 
the  assumption  of  the  existance  of  calcareous  rooks  in  depth, 
which  is  impossible  to  prove  or  disprove* 

The  conoluaion  that  the  ores  and  associated  dike 
rooks  are  of  pneumatolytic  oiigin  rests  on  a  direct  inter- 
pretation of  definite  evidence,  and  ie  equally  applicably 
whether  the  gases,  are  of  juvenile  or  resurgent  origin,  or 
both* 

The  small  amount  of  chaloocite,  which  is  developed 
entirely  in  the  bornite,  is  believed  to  be  the  product  of 
secondary  processes.  The  origin  of  an  almost  negligible 
amount  in  small  blebs  in  the  bornite  cannot  be  determined. 


88. 


Copper  Mountain,  Yale  District,  British  Columbia. 

ItfTRODUCTIOfl 

Situation.  -  Copper  Mountain  in  southern  British  Columbia 
is  a  north  -  south  ridge  rising  about  1500  feet  above  the 
Similkameen  River  about  13  miles  south  of  Princeton,  the  chief 
town  of  the  Similkameen  Mining  Division  of  the  Yale  District. 
The  mineralized  portion  of  the  mountain  is  largely  the  proper- 
ty of  the  British  Columbia  Copper  Company,  which  has  thorough- 
ly prospected  the  deposits  by  trenches  and  diamond  drills,  and 
is  rapidly  pushing  development  work.  An  attractive  camp  has 
been  established  on  the  upper  slopes  of  the  mountain  in  the  de- 
lightful open  forest  of  yellow  pine  and  lodge-pole  pine  charac- 
teristic of  this  climatic  belt.  (Pig.  5  ) . 

Summary  of_  Previous  ffork.  -  The  deposits  on  Copper  Mt. 
were  first  brought  to  the  attention  of  geologists  by  a  note 
published  by  J.  Jf.  Kemp  in  1901,   in  which  the  bornite  was 
described  as  a  primary  constituent  of  pegmatite  dikes,  and  was 
stated  to  be  of  direct  raagmatic  origin.  A  microscopic  study 

of  the  material  collected  by  Prof.  Kemp  was  made  somewhat  later 

2 
by  Jules  Catherinet  ,  who  describes  Copper  Mountain  as  a  mass  of 

gabbro,  with  other  eruptives,  cut  by  ore-bearing  seams  and  peg- 


1.  Trans.  A. I. M.S.,  Vol.  31,  p.  182, (1901). 

2.  Eng.  and  Min.  Jour.,  Vol.  79,  p.  125-127, (1905) . 


- 

' 


88A. 


PLATE  III 


5      View  looking  wast  from  Copper  Mountain,  Similkameen 
District,  British  Columbia. 


Fig     6.     View  looking  weet  from  La  Fleur  Mountain,  Danville 
Dietriot,  Washington. 


* 


89, 


matite  dilres  in  the  mineralized  area.  He  wrote  that  the  ore 
occurs  in  two  ways,  contrasted  but  genetically  akin.  In  one, 
the  sulphides  form  coatings  along  small  crevices,  accompanied 
by  little  pegmatite  seams  of  the  same  magnitude  as  ths  bornite 
veinlets.  In  the  other,  the  bornito  occurs  in  large,  typical 
pegmatite  veins  exposed  on  the  Copper  Cliff  Claim.  Bornite  in 
masses  up  to  1/2  cubic  ft.  were  described  scattered  through  the 
pegmatite.  The  bbrnita  is  accompanied  'by  smaller  amounts  of 
chalcopyrite.  Orthoclase  and  biotlte  are  the  chief  minerals, 
with  smaller  amounts  of  oligoclase,  tourmaline,  fluorite  and 
calcite  and  a  very  little  primary  quarta.   It  is  stated  that 
the  bornite  is  partially  altered  to  chalcopyrite.   3ketches 
and  descriptions  make  it  quite  evident  that  some  of  the  chal- 
copyrite is  of  secondary  nature,  but  it  seems  to  me  to  be  very 
doubtful  if  this  is  true  for  tho  larger  part.  The  chief  ar- 
gument favoring  the  magraatic  origin  of  the  bornite  is  its  oc- 
currence with  unaltered  orthoclase  and  biotite  in  pegmatite. 
Calcite  is  said  to  be  the  only  mineral  present  which  seems  to 
be  an  alteration  product.   It  was  not  observed  by  Catherinet 
to  have  any  genetic  relations  with  the  bornite.  Gold  and 
platinum  (as  sperrylite)  occurring  in  flakes  in  bornite  and 
in  orthoclase  ara  described.   Sperrylite  in  biotite  is  also 
mentioned.  Both  are  believed  to  be  magmatic. 

The  ores  have  been  described  also  by  D.  N.  Scott,  who 

1.   Jour.  Canadian  Mining  Inst.,  Vol.  5,  pp.  493-508,  1902. 


90. 


emphasises  some  structural  features.  The  geology  of  the 
region  la  treat od  in  a  very  general  way  by  Chas.  Carasell  in 
a  report  of  the  Canadian  Geological  Survey. 

The  geology  of  the  mineralized  region  on  the  surface 
of  the  mountain  has  been  worked  up  very  thoroughly  by  S.  H. 
Ball  for  tho  British  Columbia  Copper  Company,  and  certain  rock 
determinations  have  been  made  by  C.  P.  Berkoy.  Their  reports 
however  have  not  been  made  available  for-.public  uso. 

The  geologic  sequence  according  to  Hr.  Ball's  report  is 
as  follows: 

Pleistocene 

(1}  Glacial  drift; 

Eocene 

(2)  porphyry  dikes  (moiizonites,  quartz,  porphyry, 

andesites  and  others) ; 

i'ertiary  lavas  (nearest  a  couple  of  miles  away); 

Jurassic  (?) 

(3)  Pegmatite  and  granitoid  equivalents; 

(4)  Granodiorite; 

(5)  Augite  monzonite  porphyry,  and  a  gneissic  augite 

monzonite  porphyry. 

Pre-Jurassic 

(6)  Altered  rocks,  in  part  probably  altered  sediments. 

£he  sulphides  occur  in  the  pegmatites  and  in  various  rocks 
of  earlier  age.  The  Tertiary  dikes,  which  tiro  very  abundant, 
are  entirely  barren,  and  clearly  post-ore.  They  are  far  more 
extensive  than  the  mineralized  area,  and  show  no  traces  of  the 


1.  Report  986,  C.G.3.,  (The  Similkameen  District'*  1906. 


91. 


alteration  associated  with  the  sulphides. 

Hy  information  concerning  the  ores  was  gained  chiefly 
from  an  examination  along  numerous  trenches  across  the  moun- 
tain and  from  material  on  the  dumps  of  shallow  shafts.  There 
were  no  underground  workings  of  importance  at  the  time  of  my 
visit,  and  most  of  the  prospect  pits  were  flooded  and  inacces- 
sible.  I  v;as  given  the  privilege  of  examining  the  drill  records 
and  the  sections  drawn  from  them,  and  also  gained  much  valuable 
information  from  :.Er.  Patrick  Crane,  in  charge  of  the  property 
at  the  time,  and  from  Mr.  James  Tremlett,  an  engineer  of  the 
company. 

ROCKS  ASSOCIAIV.J  7ITH  TH3  ORES. 

\ 

The  sulphides  occur  most  abundantly  in  a  dark  medium- 
grained  gabbro.  The  prevailing  rock  of  the  region  is  grano- 
diorite,  and  it  ie  probable  that  the  rock  associated  with  the 
ore  is  merely  a  more  basic  phase  of  the  larger  mass.  A  speci- 
men of  the  rock  remote  from  ore  was  found  to  contain  basic 
anAesine  and  orthoclase  with  the  latter  slightly  less  abundant 
than  tha  former,  and  a  subordinate  amount  of  quartz.  The  pla- 
giocle.se  in  some  grains  ranges  from  a  core  near  acid  labrador- 
ite  in  composition  to  a  rim  near  oligoclase.  The  chief  dark 
constituents  are  pale  green  augite  and  a  somewhat  smaller 
amount  of  dark  brovra  biotite.  Apatite  and  magnetite  are  pres- 
ent as  accessory  minerals.  The  rock  is  apparently  more  basic 
than  the  usual  granodiorite.  Near  the  ore,  the  orthoclase  be- 


92. 


cornea  subordinate  or  is  entirely  lacking,  and  there  is  usual- 
ly no  quartz.  The  brown  biotite  of  the  more  remote  rock  is 
lacking,  but  its  place  is  taken  by  a  green  variety.  The  py- 
roxene remains  abundant  as  before.   The  more  basic  types  of 
this  rock  would  be  properly  termed  gabbro,  but  in  places  it 
seems  closer  to  an  augite  diorite  in  composition. 

The  plutoiiic  rocks  are  cut  by  seams  of  pegmatitic  mater- 
ial, usually  less  than  half  and  inch  in  thickness  but  occa- 
sionally in  small  dikes,  sis:  inches  or  aore  across.  In  them, 
pink  orthoclase  is  the  chief  constituent,  usually-  with  a  little 
alb it e  associated  with  it.   Quartz  becomes  of  importance  in 
places,  and  apatite  is  usually  common.  JOeep  green  biotite  is 
rather  abundant.   .   Calcite  is  usually  present,  bat  it  is  ap- 
parently an  alteration  product.   Small  irregular  segregations 
of  similar  material,  often  very  rich  in  the  biotite,  occur 
in  the  heart  of  the  rock  and  are  probably  related  to  the  peg- 
matites in  origin. 

The  older  porphyry  of  the  mountain,  termed  augite-mon- 
zonite  porphyry,  is  somewhat  similar  in  general  composition  to 
the  associated  plutonic  rocks,  and  may  be  merely  an  earlier 
product  of  the  same  intrusion.  Although  the  rock  is  greatly 
altered  by  the  mineralizing  processes /\ all  my  specimens,  it 
can  be  seen  that  the  phenoorysts  are  plagioclase  (oligoclase- 
andesine  (?)  )  and  augite  in  a  microgranitic  ground-mass  con- 
taining abundant  orthoclase  and  a.  little  quartz.  The  latter 
may  have  been  due  in  part  to  the  mineralization.  The  green- 


93. 


biotite  common  in  the  diorite  or  gabbro  occurs  in  these  rocks 
also,  The  presence  of  the  pale  augite  and  the  green  biotite 
suggests  a  relationship  to  the  plutonlc  rocks. 

The  dikes  later  than  the  ores  were  not  studied.  They  are 
fresh,  unmineralized  fine-grained  porphyries  for  the  most  part, 
among  which  a  quartz  porphyry  is  a  prominent  member. 

OSES  AUD  ROCK-ALTERATIOlf. 

The  ores  of  Copper  Mountain  are  chiefly  bornite  and  chal- 
copyrite  in  approximately  equal  amounts,  The  chalcopyrite  and 
bornite  are  contemporaneous  in  part,  but  there  is  occasionally 
a  little  evidence  of  corrosion  by  the  bornite  which  suggests 
that  its  period  of  formation  lagged  a  little  after  the  deposi- 
tion of  the  phalcopyrite  had  ceased.  A  small  amount  of  pyrr- 
hotite  was  noted  in  ores  from  the  Silver  Dollar  claim  on  the 
northern  end  of  the  mineralized  zone.  Pyrite  is  noticably  ab- 
sent.  'Jagnetite  occurs  but  is  not  abundant.   In  ores  from 
Voigts1  Camp  a  mile  or  so  from  the  mountain,  specularite  is  a 
prominent  mineral,  but  it  does  not  occur  in  notable  amounts 
with  the  bornite  ores  of  the  larger  deposit.  A  few  small 
blebs  of  galena  were  detected  under  the  microscope,  but  the 
mineral  is  of  no  quantitative  importance.  The  sulphides  are 
disseminated  through  the  gabbro  or  diorite,  or  occur  as  thin 
seams  along  slight  fractures.  The  latter  are  prominent  in 
broken  ore,  for  the  rock  commonly  splits  along-  such  cracks, 
leaving  the  bornite  as  a  thin  film  on  the  face  of  the  block. 


94, 


In  the  older  altered  porphyries  and  sediments,  the  ore- 
minerals  appeared  to  me  to  be  less  abundantly  distributed 
and  more  cloaaly  confined  to  partings  or  sea^a.  She  sul- 
phides also  occur  in  the  small  seams  or  dikes  of  pegroatitic 
material,  and  in  the  mica  rich  segregations.  Although  fair- 
ly coarse  grains  of  bornite  or  chalcopyrite  are  sometimes  seen 
in  these  associations,  tho  sulphides  in  the  pegrnatitic  materi- 
al are  of  little  importance  from  a  quantitative  standpoint  and 
constitxite  only  a  small  fraction  of  the  ore.   The  copper  values 
lie  ohiei'ly  in  the  mineralized  gabbro  and  older  rocks. 

There  is  little  rock  alteration  associated  with  the  sul- 
phides in  the  pegmatite.   The  ore-minerals  are  later  than  the 
feldspar  and  biotite,  uhich  they  corrode  or  cut  in  veins,  but 
their  introduction  is  not  accompanied  by  sericite,  chlorite 
or  epidote,  w—  minerals  which  are  commonly  associated  with 
sulphides  of  hydrothermal  origin.   Calcite  is  often  abundant, 
but  appears  to  be  later  than  the  ores. 

In  the  plutonic  rocks  and  the  older  porphyries,  however, 
there  is  fairly  pronounced  alteration  where  the  sulphides  are 
abundant.  Th>     ins  of  oornite  or  chalcopyrite  are  commonly 
bordered  by  rims  of  epidote,  which  is  often  developed  in  small 
amounts  elsewhere  through  the  rock.   The  plagioclase,  especial- 
ly the  basic  cores  of  the  grains,  are  usually  somewhat  serici- 
tised;  the  orthoclase  on  the  other  hand  is  little  altered. 
Chlorite  ia  not  abundant,  although  it  occurs  to  some  extent  in 
the  basic  constituents.   Calcite  is  wide-spread,  cutting  the 


'•95, 


rock  in  many  small  seams,  or  scattered  as  replacements  of 
the  rook-minerals.  It  is  probably  a  product  of  the  closing 
phases  of  the  mineralization. Bornite  in  small  amounts  was 
observed  in  one  oalcite  seam,  which  indicates  that  it  contin- 
ued to  form  in  a  feeble  way  even  under  these  late  milder  con- 
ditions. 

OZIDATIOH  AND  MRICHMEHT 

The  surface  alteration  is  very  slight.  There  is  a  little 
carbonate  staining,  but  it  usually  does  not  penetrate  many 
feet  below  the  surface.  The  mountain  has  been  severely  glacia- 
ted, and  all  traces  of  an  earlier  gossan,  if  such  existed,  have 
been  removed.  The  ground-water  level  is  between  ten  and  twenty 
feet  below  the  surface  in  most  places,  and  the  secondary  sul- 
phides are  only  feebly  developed. 

Little  of  the  bornite  exposed  to  view  is  entirely  free 
from  traces  of  the  secondary  sulphides,  but  in  no  place  ia 
the  enrichment  of  especial  value.  The  development  of  veinlets 
of  chaloocite  and  covellite  through  the  bornite  is  the  chief 
manifestation  of  surface  changes.  These  minerals,  however,  are 
also  accompanied  by  chalcopyrite,  which  has  developed  as  fine 
lattices  of  intersecting  plates,  closely  similar  to  the  rela- 
tions described  in  the  ores  from  La  Pleur  Mountain.  The  plates 
of  secondary  chalcopyrite  range  from  fairly  coarse  plates, 
scattered  here  and  there  along  veinlets  which  are  easily  detec- 


. 


line  erifr 


96. 


ted  under  low  magnifications,  to  the  finest  grills  of  thin 
intersecting  lines  on  the  polished  surface  which  can  hardly 
be  detected  under  the  oil-immersion  lens.  As  in  the  other 
case  described,  it  seems  very  probable  that  submicroscoplo 
ohalcopyrite  exists  in  this  form  which  may  account  for  the 
yellowish  tints  of  some  of  the  bornite  grains.  The  secon- 
dary origin  of  the  visible  chalcopyrite  plates  is  convincing- 
ly shown  by  the  dependence  in  distribution  upon  veinlets  of 
ohalcocite  and  oovellite. 

Chalcooite  in  the  graphic  structure  with  bornite.  Be- 
sides the  chalcocite  of  easily  recognized  replacement  origin 
derived  by  secondary  processes,  just  described,  the  mineral 
also  occurs  in  small  amounts  associated  with  the  bornite  in 
groups  of  irregular,  curved  and  divided  blebs  and  lobes, 
which  possess  a  rather  definite  pattern  not  unlike  that  of 
certain  metallic  eutectios  or  of  mioropegmatitic  intergrowths 
of  quartz  and  feldspar.  The  relation  has  been  termed  the 
graphic  structure.  In  the  bornite  from  Copper  Mountain,  the 
chaleocite  in  these  structures  is  differentiated  from  the 
secondary  sulphides  previously  mentioned  by  the  disregard  it 
shows  for  the  obvious  channel-ways  which  would  be  sought  by 
replacing  solutions,  such  as  grain  boundaries,  gangue  con- 
tacts, or  small  cracks.  For  the  most  part,  it  shows  little' 
regularity  which  would  suggest  the  influence  of  orystallo- 


1.   L.  C.  Graton  &  D.  H.  McLaughlin,  Econ.  Geol.,vol. 
12,  p.  23,  1917. 


OltJOOB 


97. 


graphic  forces  in  its  formation,  but  in  a  few  examples,  there 
is  a  rough  allignment  which  may  possibly  indicate  such  ten- 
dencies. These  graphic  structures  in  the  Copper  Mountain  ores 
are  usually  somewhat  modified  by  chalcocite  replacement,  but 
the  latter  appears  to  be  related  to  the  chalcooite  of  the  vein- 
lets,  and  superimposed  on  an  older  structure.  The  chalcocite 
in  both  structures  is  bluish  and  no  line  can  be  drawn  between 
them  where  they  intersect.  However,  the  existance  of  graphic 
ohalcocite  in  some  blebs  unassociated  with  the  material  clear- 
ly of  replacement  origin,  suggests  that  it  is  not  dependent  on 
the  feeble  veinlets  of  chalcocite  which  represent  enrichment 
from  the  present  surface. 

The  origin  of  the  chalcocite  in  these  graphic  structures 
is  one  of  the  most  difficult  problems  encountered  in  this  in- 
vestigation. Further  discussion  will  be  postponed  until  later 
pages,  where  wider  evidence  will  be  available  and  the  many  ar- 
guments bearing  upon  its  solution  can  be  presented  in  full. 

Discussion  and  Summary.  The  sulphides  in  the  pegmatite 
dikes  may  be  considered  primary  constituents,  but  as  they  are 
later  than  the  silicates  with  which  they  are  associated,  it  is 
probable  that  they  were  formed  under  pneumatolytio  rather  than 
strictly  magmatic  conditions.  The  occurrence  of  tourmaline 
and  fluorite  in  these  ores  as  mentioned  by  Catherinet,  streng- 
thens this  interpretation  of  origin.  The  more  widely  developed 
ores  in  the  gabbro  and  older  rocks  may  be  attributed  with  a 
fair  degree  of  certainty  to  the  same  general  source,  but  the 


. 


,j 


' 


got 


98, 


close  association  of  epi dote  and  sericite  with  the  sulphides 
in  most  parts  of  the  deposit  indicates  that  the  sulphides 
continued  to  form  and  were  deposited  in  greatest  abundance 
under  later  hydrothermal  conditions, 

A  small  amount  of  ohalcocite  occurs  in  graphic  structures, 
and  a  little  in  veinlets  of  undoubted  secondary  origin.  With 
the  latter  is  associated  a  small  amount  of  chalcopyrite  in 
lattice  forms,  some  exceedingly  fine-grained,  and  a  littlo 
covellite. 


eeJ 


99 


1 
-Bngels.  California* 


INTRODUCTION  AND  CONCLUSIONS 

The  Engels  copper  mine,  located  in  northern  Plumas  Coun- 
ty, California,  27  miles  northeast  of  Xeddie  on  the  Western 
Pacific  Railroad,  lies  at  an  elevation  of  about  5,200  feet  on 
a  gentle,  forested  slope  that  descends  toward  the  southwest 
from  the  crest  of  one  of  the  eastern  ridges  of  the  Sierra  Ne- 
vada. The  property  has  been  worked  more  or  less  steadily  for 
many  years,  but  has  suffered  because  of  ramoteness  from  rail 
transportation  and  of  difficulty  in  either  smelting  or  concen- 
trating the  bornite-chalcopyrite  ore  which  it  chiefly  yields. 
The  installation  of  an  oil-flotation  plant  and  a  vigorous  cam- 
paign of  underground  development  have  recently  brought  produc- 
tion to  a  higher  level  than  previously  attained. 

Preceding  these  recent  developments,  a  geological  examin- 
ation of  the  deposit  was  made  by  Mr.  H.  W.  Turner,  and  speci- 
mens which  he  collected  were  studied  microscopically  by  Prof. 

A.  P.  Rogers,  of  Stanford  University.  The  results  of  their 

2 
investigations,  published  as  a  joint  paper,  merit  particular 


1.  Taken  with  little  change  from  an  article  entitled  Ore 
Deposition  and  Enrichment  at  Bngels,  California,  by  !••  C. 
Qraton  and  D.  H.  McLaughlin,  published  in  Kcon.  Geol.,  Vol. 
XII,  pp.  1-38.  Jan.  1917. 

2.  Scon.  Geol.,  Vol.  9,  pp.  359-391 ,( June,  1914). 


100 


attention  because  of  the  two  principal  conclusions  which  the 
authors  reach,  viz.:  (1)  that  the  ores  are  of  direct  magmatio 
origin,  and  (2)  that  the  development  of  ohalcooite  and  some 
oovellite  by  replacement  of  bornite  is  the  work  of  ascending, 
heated  alkaline  waters  and  is  thus  to  be  regarded  as  an  exam- 
ple of  "upward  secondary  enrichment." 

The  accelerating  tendency  in  recent  years  to  ascribe  the 
origin  of  many  ore  deposits  to  igneous  influences  must  be  ad- 
mitted to  rest  more  on  reasonable  inference  and  on  elimination 
of  objections  than  upon  direct  and  positive  evidence.  Any  in- 
stance in  which  sulphide  ores  have  been  formed  in  syngenetio 
relation  to  the  enclosing  igneous  rook  therefore  assumes  spec- 
ial interest  and  significance.  The  importance  thus  given  to 

the  Engels  occurrence  was  only  increased  by  a  paper  by  T«  T. 

1 
Head,  who,  after  a  brief  visit  to  the  deposit,  concluded  that 

the  magmatic  origin  of  the  ores  is  not  certain* 

Still  more  interest,  if  possible,  attaches  to  the  Sngels 
deposit  in  connection  with  the  origin  of  the  chalocite.  The 
idea  of  "upward  secondary  enrichment,"  though  it  had  been  ear- 
lier hinted  at  or  somewhat  vaguely  invoked  to  explain  certain 

observations,  was  proposed  as  a  definite  hypothesis  for  copper 

2 
sulphide  ores  in  an  earlier  paper  by  Professor  Rogers.  That 

paper  was  virtually  an  announcement  of  the  hypothesis  and  an 


1.  Mining  and  Scientific  Press,  July  31,  1915,  pp.  167- 
171. 

2.  Eoon.  Geol.,  Vol.  8,  pp.  781-794,  Dec.,  1913. 


» 


.       .?     ';•• 


100A. 


PLATE  IV 


Fig.  7.  The  Engela  Mine  California,  showing  the  various 
tunnel  antranoee. 


Fig.  t.  The  outcrop  of  the  Engela  Vine  California,  above 
upper  tunnel. 


lo; 


application  of  it  to  the  deposits  of  Butte,  in  advance  of 
the  complete  description  of  an  unnamed  occurrence  which, 
in  Professor  Rogers 'a  opinion,  afforded  the  most  convincing 
proof  of  the  hypothesis.  This  hypothesis  was  immediately  ac- 
cepted and  adopted  by  the  corps  of  able  and  assiduous  Inves- 
tigators at  Stanford  and  has  already  been  given  by  them  a  con- 
spicuous place  in  the  literature.  Among  interested  students 
elsewhere,  the  expected  article  by  Professor  Rogers  that  should 
deal  with  specific  evidence  of  upward  secondary  enrichment  in 
a  carefully  studied  example  was  awaited  with  interest;  it 
proved  to  be  the  paper  on  the  Engels  mine. 

In  a  later  article,  Professor  Eogers  had  somewhat  revised 
his  first  ideas,  especially  in  recognizing  that  part  of  the 
Sngels  chalcocite  is  due  to  the  normal  process  of  descending 
enrichment.  In  consequence,  some  of  our  evidence  upon  this 
important  point  is  merely  confirmatory,  as  is  indeed  the  case 
with  regard  to  wmy  of  the  other  matters  discussed.  There  re- 
mains, however,  sufficient  divergence  of  views  as  in  our  opini- 
on to  warrant  presentation  of  all  our  evidence. 

With  regard  to  the  two  principal  questions  involved  in 
the  JSngels  occurrence,  we  conclude  as  follows: 

1,  She  ores,  instead  of  being  magmatic  in  the  sense 
that  they  were  initial  constituents  of  the  dioritic  rock  in 
which  they  occur,  were  introduced  after  the  rook  had  solidi- 
fied and  had  suffered  notable  dynamic  and  chemical  changes, 


102 


and  constitute  replacements  formed  under  pneumatolytio  and 
hydrothermal  conditions.  The  deposition  of  the  sulphides 
was  an  intermediate  episode  in  a  complex  and  unusually  com- 
plete sequence  of  magraatio  phenomena  and  after-effects  that 
began  with  crystallization  of  the  parent  rock  and  ended  with 
the  formation  of  zeolites  and  carbonates. 

2.  Although  the  possibility  of  formation  of  a  small 
amount  of  chaloocite  from  ascending  solutions  cannot  be  ab- 
solutely excluded,  no  satisfactory  evidence  of  chalcocite  of 
replacement  origin  formed  in  this  way,  i.e.,  by  "upward  secon- 
dary enrichment,"  has  come  under  our  observation.  Most  of  th« 
chalcocite  and  all  of  the  covellite  at  Engels  unquestionably 
result  from  replacement  of  earlier  sulphides  through  the  agen- 
cy of  descending  metorio  waters,  and  a  competent  explanation 
for  all  of  the  ohalcocite  is  to  be  found  in  normal  downward 
enrichment. 

OEHHRAL  DESCRIPTIVE  TREATMENT  -  COUNTY. 
ROCK,  ORES,  AND  ALTERATION, 

Magmatio  Period. 

Solidification  of  Horite.  -  The  Engels  ore-deposit  occurs 
in  a  body  of  rather  dark,  medium-grained  plutonic  rock,  which 
is  probably  a  basic  differentiate  of  the  great  Sierra  Nevada 


103 


batholith  of  granodiorite.  The  rock  was  nowhere  seen  in 
its  original  condition,  but  it  may  confidently  be  inferred 
from  its  altered  phases  that  it  was  noritio  in  character, 
being  composed  of  plagioolase  and  slightly  subordinate  am- 
ounts of  orthorhombic  and  monoclinic  pyroxenes,  and  biotite. 
The  feldspar  varies  somewhat  in  character,  but  usually  it  is 
near  an  acid  labradorite  in  composition  (Ab^  An^).  Bronzite, 
hypersthene,  and  diopside  are  present  in  quantity  that  approx- 
imately equals  the  feldspar,  while  the  biotite  is  usually 
subordinate  in  amount,  although  its  small  laths  are  occasion- 
ally plentiful.  Zircon,  apatite  and  euhedral  magnetite  are 
common  accessories. 

The  form  of  the  body  of  no  rite  is  not  definitely  known,, 
as  surface  croppings  are  scarce,  and  its  contacts  with  the 
surrounding  rook  of  the  batholith  can  not  be  traced.  Under- 
ground, en  several  levels,  in  passing  out  of  the  ore,  sili- 
ceous rocks  are  encountered,  which  possibly  mark  the  limits 
of  the  norite  body  in  those  directions.  In  a  cross-cut  off 
Tunnel  4,  one  of  these  light-colored,  acidic  rocks  is  en- 
countered, and  proves  to  be  a  granodiorite  porphyry.  The 
rock  is  characterized  by  large  feldspar  phenocrysts,  set  in 
a  medium  to  fine  grained  ground-mass,  consisting  chiefly  of 
unstriated  feldspar  and  biotite.  Under  the  microscope,  the 
"phenocrysts"  are  seen  to  be  composite,  consisting  of  aggre- 
gates of  smaller  laths  of  andesine,  of  about  tie  same  grain 
as  the  constituents  of  the  ground-mass  (Pigs. 53  and51+) . 


. 

£ii-  r 

cJOiiiGxfo  ei  " 


The  ground -mass  is  ohiefly  orthoelase,  quartz  and  blot it e 
with  an  accessory  amount  of  plagioclase*  This  somewhat  acid 
rock  may  reasonably  be  interpreted  as  another  variant  of  the 
granodiorlte  batholith  bat  of  subordinate  importance. 

Pneumatolytic  Period, 

Alteration  of  Mo-rite*  -  After  the  crystallization  of 
the  norite,  a  period  ensued  in  which  conditions  of  equilib- 
rium were  apparently  disturbed,  probably  by  increasing  con- 
centration of  water  and  other  mineralizers;  this  may  be 
termed  the  pneumatolytlc  period*  The  magmatio  products  were 
partially  altered  into  minerals  more  stable  under  the  new 
conditions;  chief  among  these  is  amphibole,  formed  in  part 
at  the  expense  of  the  pyroxenes,  while  the  labradorite  under- 
went partial  recryatallization  with  attendant  cracking  and 
straining,  and  the  production  of  rims  of  more  acidic  varie- 
ties, oligoclase  and  alb it e.  The  resulting  rock  may  be  re- 
garded as  a  peculiar  type  of  diorite,  or  perhaps  more  proper- 
ly as  a  meta-norite.  This  derivative  of  the  original  norite 
is  more  extensive  than  the  ore-bodies  and  constitutes  the 
principal  country  rock  of  the  Sngels  mine.  The  amphibole  ±a 
chiefly  a  light  green  hornblende,  but  locally,  along  seams 
and  in  segregations,  laths  of  actinolite  were  formed*  The 
hornblende  is  widely  developed  throughout  the  rock,  but  it 
is  more  abundant  and  in  largest  crystals  in  the  ore-bearing 
portions.  It  forms  large,  strikingly  poikilitic  crystals. 


si 


. .  • ,    . 


105 


frequently  containing  corroded  cores  of  pyroxene,  but  ex- 
tending far  beyond  the  original  limits  of  the  pyroxene  crys- 
tals, and  including  numerous  rounded  grains  of  labradorite 
and  small  particles  of  the  accessory  minerals  (apatite  and 
magnetite) •  Slightly  corroded  laths  of  biotite  are  fre- 
quently included  in  the  large  crystals*  In  many  cases,  the 
inclusions  are  so  abundant  that  their  total  volume  la  con- 
siderably greater  than  that  of  the  amphibole  host* 

Formation  £f  Segregations*  -  The  formation  of  small 
lens-shaped  segregations  in  the  norite  (or  diorite)  is  clear- 
ly related  to  this  period  of  pneumatolytic  alteration.  They 
probably  represent  the  products  of  the  late  crystallisation 
of  small  localized  concentrations  or  residues  after  the  crys- 
tallization of  the  normal  magmatic  minerals*  In  these  segre- 
gations, actinolite  and  albite  are  the  chief  minerals,  al- 
though in  a  few  cases,  biotite  assumes  an  equal  importance. 
In  one  interesting  occurrence,  a  lenticular  mass,  over  eight 
inches  in  diameter  and  approximately  two  inches  thick,  occurs 
In  the  fine-grained  diorltic  rook.  The  outer  layer  of  this 
segregation  consists  of  stubby,  green  laths  of  aotinolite, 
and  it  Is  succeeded  Inward  by  a  well-marked  zone  of  albite, 
in  coarse  anhedra.  This  in  turn  surrounds  an  inner  core 
which  was  originally  an  aggregate  of  actinolite,  albite  and 
large  crystals  of  titanite.  In  this  and  in  most  of  the  other 
segregations  studied,  the  cores  and  frequently  the  outer 
layers  are  largely  replaced  by  chlorite,  epidote  and  oaloite, 
with  associated  copper  ores,  but  these  changes  are  clearly 


106 


subsequent  to  the  formation  of  the  segregations  themselves. 
The  amphibole  is  always  earlier  than  the  alblte,  the  latter 
being  molded  about  the  well- formed  prisms  of  the  former  in 
many  oases,  but  it  is  later  than  the  biotite,  except  that 
locally,  where  the  biotite  is  especially  abundant  in  a  segre- 
gation, the  order  may  be  reversed. 

Formation  of  Pegmatites.  -  In  a  few  places  underground, 
the  workings  expose  narrow  pegmatite  dikes  cutting  the  diorite. 
They  are  composed  chiefly  of  acid  feldspar  and  subordinate 
quartz,  with  a  small  amount  of  actinolite,  usually  concentra- 
ted near  the  edges  or  developed  in  the  adjoining  wall  rook. 
Chalcopyrite,  but  rarely  bornite,  is  associated  with  the  peg- 
matites, as  intermittent  central  bands,  or  as  disseminated 
grains  in  the  neighboring  diorite*  In  one  specimen  from  a 
one-inch  dike  exposed  in  a  drift  to  the  northeast  off  Tunnel 
4,  microscopic  examination  shows  the  feldspar  to  be  albite- 
oligoclase,  with  small  cores  of  a  more  basic  type.  With  it 
are  pale  green  laths  of  actinolite,  with  subordinate  amounts 
of  quartz,  titanite,  and  apatite.  A  persistent  band  of  chal- 
oopyrite  along  the  center  of  the  dike  is  the  only  sulphide  in 
this  case.  In  another  specimen  from  a  larger  dike,  the  peg- 
matite was  found  to  be  composed  of  quartz,  microcline,  and 
small  amounts  of  labradorite,  titanite,  and  a  deeply  pleoch- 
roic  hornblende,  with  a  few  crystals  of  tourmaline.  Patches 
of  ohalcopyrite  and  bornite  occur  in  it,  and  appear  to  be 
original  constituents. 


•v«       "?-.- 


Dynamic  Changes •  -  A  pronounced  straining  and  shearing  of 
the  feldspars,  accompanied  by  recrystallization  in  various 
degrees,  is  notable  throughout  the  rocks  studied.  In  many 
cases,  no  traces  of  these  changes  are  found  in  the  large 
amphibole  crystals,  although  the  surrounding  and  included 
feldspars  show  wavy  extinction  (see  Fig. 52)  and  partial 
granulation*  In  other  oases,  the  amphibole  itself  may  lose 
its  fresh,  coarse-grained  character,  and  become  reduced  to 
shreddy  masses  of  variously  oriented  fibers,  often  distinct- 
ly uralitic  in  appearance.  It  may  therefore  be  inferred 
that  the  straining  and  accompanying  reerystallization  took 
place  within  the  pneumatolytic  period,  although  locally  it 
may  have  continued  longer,  or  even  have  started  somewhat 
later.  The  reerystallization  is  not  confined  to  the  diorite, 
but  is  also  well  shown  in  the  neighboring  siliceous  rocks. 
An  excellent  example  is  the  breaking  dov/n  of  the  phenocrysts 
of  the  granodiorite  porphyry  into  an  aggregate  of  andesine 
crystals,  as  is  clearly  shown  in  the  two  photographs  (Figs.  53 
and 54-)  •  In  several  of  the  specimens,  a  distinct  schistose 
structure  has  been  produced  by  the  grouping  of  the  recrys- 
tallisod  biotite  laths  in  bands.  2he  feldspar  assumes  a 
crystalloblastic  texture  in  these  cases,  but  the  grains  of 
the  pyroxenes  show  little  change.  Where  the  recrystalliza- 
tion is  most  intense,  apatite  is  abundant,  forming  elonga- 
ted clusters  of  rounded  grains  of  distinctly  different  habit 
from  the  ordinary  niasrraatic  apatite.   In  one  or  two  slides,  a 
subparallel  arrangement  of  plagioclase  laths  may  be  attribu- 


10* 


tod  to  the  flowage  of  the  partially  crystallized  magma,  but 
superimposed  on  this  structure  are  changes  similar  to  those 
previously  described,  which  can  be  satisfactorily  explained 
only  as  the  result  of  stresses  acting  on  the  rigid  rock. 
The  fractured  apatite  crystal  in  ]?ig,52  is  striking  evidence 
of  dynamic  action. 

Development  of  Iron  Oxides •  -  Associated  with  the  more  in- 
tense phases  of  the  development  of  the  amphibole,  but  for  the 
most  part  a.  littlo  later,  is  magnetite,  in  large  masses  with 
abundant  incluaiona  of  apatite.  This  iron  ore  is  clearly  later 
than  the  small  euhedral  grains  of  magnetite,  accessory  in  the 
norite,  which  were  included  by  the  growing  crystals  of  the 
pneuraatolytic  hornblende.  The  later  magnetite  occurs  commonly 
molded  about  the  aaiphibole  forms,  occasionally  slightly  corrod- 
ing them,  and  in  a  few  oases  it  is  so  abundant  that  the  crys- 
tals of  hornblende  appear  as  if  set  in  a  cement  of  magnet it* 
and  apatite,  with  a  relatively  subordinate  amount  of  rounded 
feldspar  grains  and  a  little  spinel  (see  Pig.59).  The  pneu- 
matolytic  origin  of  this  magnetite  is  indicated  by  its  close 
relationship  with  the  development  of  the  amphibole,  and  thia 
view  ia  confirmed  by  the  abundant  apatite  and  by  the  occur- 
rence in  several  instances  of  small  crystals  of  tourmaline 
in  close  association  with  the  magnetite. 

With  the  magnetite,  and  perhaps  in  part  a  little  later 
in  origin,  ocour  ilmonite  and  hematite.  Intergrowths  of  all 
throe  minerals  have  boon  observed,  ?;hile  those  of  ilraenite 
and  hematite  are  very  common,  occurring  in  nearly  all  of  the 


109 


polished  chips  studied.  In  theae  intergrowtha  the  hematite 
id  usually  auoordinate,  and  appears  moat  commonly  on  the 
polished  surface  as  fine  white  linea,  in  parallel  orientation 
in  one,  or  more  rarely,  two  directions.  Other  sections  show 
the  hematite  us  elongated,  narrow  ovals  or  aa  dots  scattered 
with  a  rough  regularity  through  the  ilmenite*  2he  magnetite 
occasionally  contains  platen  of  hematite,  and  now  and  then 
occurs  in  rather  patchy  inclusions  in  ilmenite -hematite  areas, 
the  fine  white  lines  of  hematite  passing  uninterruptedly 
through  the  magnetite  as  well  as  the  ilmenite*  (These  minerals 
were  determined  by  mineralographic  methods,  and  the  presence 
of  ilnenite  was  confirmed  by  obtaining  titanium  tests  from 
gravity  concentrates  from  the  crushed  specimens* 

Development  of  Pneumat oly t ic  Sulphides*  -  Che  most  impor- 
tant phase  of  mineralization,  as  far  as  the  value  of  the  ore 
io  ccnoorned,  is  that  which  is  characterized  by  the  develop- 
ment of  chalcopyrite  and  bornite.  In  most  oases,  these  sul- 
phide ores  are  accompanied  by  notable  hydrothermal  alteration 
of  their  host  and  are  therefore  described  under  a  subsequent 
heading,  but  in  a  few  specimens,  they  occur  in  reruarkably 
fresh  feldspathic  rock.  U?hey  are  clearly  later  than  the  rook 
minerals,  which  they  surround  and  slightly  corrode,  and  are 
in  places  so  abundant  that  the  rounded  grains  of  plagioclas* 
and  laths  of  biotite  appear  as  if  set  in  an  opaque  cement 
(Fig»5l),   In  relative  smoothness  of  outline,  the  sulphide 

1.   J.  Murdoch,  Loc.  olt. 


110 


grains  rosemble  those  of  magnetite.  These  sulphides  seam  to 
be  analogous  to  the  ores  in  the  pegmatite,  previouoly  men- 
tioned, and  are  regarded  as  the  earliest  in  origin  whioh  w« 
have  observed.  The  structure  might  readily  suggest  a  mag- 
matio  origin  for  the  chalcopyrite  and  bornite,  but,  on  more 
careful  examination,  occasional  laths  of  chlorite  and  seri- 
cite  may  be  seen  included  in  the  sulphide  grains,  and,  where 
the  sulphides  are  in  contact  with  the  plagioolase,  thin  bor- 
ders of  alb it e  are  found.  In  the  material  whioh  best  exhib- 
its these  relations,  the  bornite  occurs  as  definite  seams, 
near  whioh  the  impregnation  of  the  rook  with  ore  is  greatest. 
Along  these  seams  the  rock  readily  breaks  apart,  yielding 
fracture  faces  completely  coated  with  bornite. 

This  pneumatolytic  stage  is  the  nearest  approach  to  mag- 
matic  conditions  that  is  shown  by  the  development  of  the  sul- 
phides, for  the  earlier  minerals  such  as  the  labradorite  and 
biotite  remain  fairly  resistant  to  the  introduced  material, 
but  the  presence  of  the  hydrothermal  minerals,  and  the  frac- 
turing of  the  rock,  indicate  that  strictly  magmatic  conditions 
had  passed. 

BJTEHSE  HYDROTHEHMAJi  PERIOD, 

Development  £f  Chlorite,  Serioite  and  ffpidote.  -  The 
commonest  type  of  sulphide  ore,  however,  is  indicative  of 
slightly  milder  conditions  than  those  of  the  pheumatolytic 
period  just  described.  Chlorite,  sericite,  epidote  and  cal- 


Ill 


cits  become  more  und  ;uore  important.  The  chlorite  is  especial- 
ly characteristic,  occurring  as  blunt  latha  or  slivers  included 
in  the  bornite  or  chalcopyrito,  or  occasionally  as  complicated 
tangles  of  small  laths,  with  the  sulphides  both  as  acutely  angn- 
lar  areas  between  them  and  as  thin  wedges  extending  along  the 
cleavage  of  the  chlorite.  Lowers  first  regarded  the  chlorite 
as  earlier  than  the  bornite  but  in  his  later  paper  he  states 
that  the  chlorite  is  younger  than  the  bornito  and  replaces  it. 
Our  evidence  ia  in  essential  accord  with  the  first  view  and  we 
are  unable  to  follow  Hogers's  argument  in  favor  of  the  second. 
In  some  cases,  the  bornite  distinctly  corrodes  the  chlorite 
laths  (Pig.  66).    Moreover,  it  is  generally  true  that  the  di- 
rection of  the  chlorite  laths  is  not  in  accord  with  the  struc- 
txire  of  the  bornite  as  rovealed  by  natural  alteration  or  by 
etching.  On  the  other  hand,  many  of  the  laths  appear  unattacked, 
and  probably  nre  virtually  contemporaneous  with  the  bornite. 
The  chlorite  was  frequently  found  between  the  magnetite  indiri- 
duuls,  somo  of  vrhich  are  well  crystallised  (Jigs. 57  and5#);  it 
was  never  obcerved  as  inclusions  in  the  magnetite.  The  biotite 
is  partially  or  completely  replaced  by  chlorite,  especially 
whore  the  latter  is  well  developed  v/ith  the  ores.  She  amphibole 
also  suffered  a  partial  alteration  to  chlorite,  and  in  many 
cases  amphibole  crystals  are  fringed  with  ragged  borders  of 
chlorite  and  bornito,  or  are  penetrated  by  voinlcte  of  these 
two  minerals.  2he  largo  hornblende  crystals  in  i'ig^y-  II  il- 
lustrate this  type  of  alteration. 


112 


Tho  sorioite  ia  largely  confined  to  the  basic  plagioolase, 
but  occasionally  it  is  more  widely  developed,  and  bears  a  re- 
lation to  the  bornlte  very  similar  to  that  shown  by  the  chlor- 
ite. Fine  laths  and  slivera  of  sericite  are  included  in  the 
bornite  or  chalcopyrite,  usually  near  the  edges,  less  commonly 
in  the  heart  of  the  grains*  In  both  of  Rogers 's  papers,  the 
sericite  laths  penetrating  the  bornite  are  regarded  as  having 
replaced  the  sulphide,  but  wo  have  failed  to  find  evidence  sup- 
porting this  view.   It  seems  to  be  much  more  in  accord  with 
the  U3i«al  criteria  of  sequence  to  regard  then,  like  the  chlor- 
ite, as  inclusions  in  the  bornite.  Apparently  the  bornite  and 
the  sericite  developed  at  about  the  same  time;  the  bornite  com- 
monly started  first,  accounting  for  the  usual  absence  of  seri- 
cite in  the  cores,  but  toward  the  edges  of  the  grains,  the  se- 
ricite formed  as  laths  a  little  ahead  of  the  last  outermost 
parts  of  the  bornite,  in  which  they  are  therefore  included. 
Soricite  ie  not  abundant  in  specimens  which  on  other  grounds 
aro  regarded  as  examples  of  the  earliest  sulphides,  and  this 
is  in  accord  with  the  view  just  expressed.  3Jho  basic  feld- 
spars are  most  thoroughly  altered  to  sericite,  vrtiile  the  more 
acid  rims  or  rocrystallized  portions  usually  remain  untouched* 
The  biotite  is  slightly  attacked  by  the  soricite,  but  to  a 
greater  extent  it  is  altered  to  chlorite,  ae  is  conclusively 
shown  by  the  presence  of  residual  sagenite  webs  in  certain 
large  areas  of  chlorite.  In  a  few  instances,  the  lath-shaped 
inclusions  of  chlorite  and  soricite  in  the  ores  may  likewise 


\ 


113 


have  bean  derived  from  corroded  biotite  or  remnants  of  amphi- 
bole,  but  this  explanation  will  not  hold  for  the  majority  of 
ciiBes  since  the  inclusions  are  in  general  distinctly  different 
in  habit  from  the  original  brown  biotite  or  the  amphibole  of 
the  rock.  Although  It  iu  impossible  to  roach  a  positive  con- 
clusion concerning  the  relative  ages  of  the  chlorite  and  seri- 
cite,  there  is  a  suggestion  that  part  of  the  chlorite  is  a 
little  older  than  the  soricite,  for  the  kind  of  corrosion  by 
bornite  shown  by  chlorite  laths  has  not  boon  certainly  ob- 
served in  tho  caoo  of  the  sericite;  also  the  chlorite  is  less 
noticeably  confined  to  the  outer  margins  of  the  bornite  grains 
than  is  the  sorioite. 

Epidote  is  also  a  common  aaaooiate  of  the  sulphides*  In 
3one  cases  voinlets  of  it  out  chlorite  (i?ig.63),  "°^  f°r  the 
most  part  the  two  minerals  aro  apparently  contemporaneous.  She 
epidote  aiay  form  fringes  about  grains  of  bornite  and  cnalcopy- 
rite;  elsewhere  it  occurs  in  euhedral  forms  about  whioh  the 
sulphides  are  molded  (j?ig.56).   In  aiany  places,  the  amphibole 
i.3  greatly  altered  to  opidote,  and  veinleta  or  irregular  pat- 
ches of  the  lattor  are  coirmon  as  roplaceraents  in  the  feldspars. 
A  few  veinleta  have  boon  observed  cutting  magnetite.  Kagged 
areas  of  fine-grainod  epidote,  thickly  filled  with  s/uall 
grains  of  bornite  or  chalcopyrito,  and  with  more  or  less  chlo- 
rite are  found  here  and  there  in  greatly  altered  speciiae:.  . 

gevelop.'aent  of  Hydrother7::al  3ulphidoa«  -  The  coniraerciul 
oro  at  Kngels  is  distributed  in  oomowhat  indefinitely -bounded 
shoots,  of  which  several  have  bean  opened  and  v/hich  collective- 
ly constitute  a  deposit  of  roughly  elliptical  plan  within  the 


11M- 


modified  no rite*  As  a  rule  the  ore  merges  into  the  country 
rook  and  all  gradations  exist  from  scattered  minute  sulphide 
grains  to  heavy  masses  of  ore  with  subordinate  gangue, 

The  bornite  and  ohaloopyrite,  which  are  the  most  abun- 
dant ore  minerals  of  the  Bngels  deposit,  were  formed  mainly 
under  hydrothermal  conditions,  together  with  a  small  amount 
of  enargite,  tetrahedrite  and  sphalerite.  Magnetite,  il- 
menite  and  hematite,  which  apparently  had  continued  to  devel- 
op during  the  early  portion  of  the  period  of  bornite  forma- 
tion, ceased  to  form  as  conditions  became  milder,  and  became 
surrounded  or  rarely  veined  by  the  bornite  and  ohaloopyrite 
which  continued  to  increase*  The  bornite  is  about  three  or 
four  times  as  abundant  as  the  ohaloopyrite* 

For  the  most  part,  the  contacts  between  the  chalcopy- 
rite  and  bornite  offer  little  evidence  of  the  sequence  of  the 
two  minerals.  Broad  bays  of  ohaloopyrite  extend  into  the 
bornite  or  smooth  projections  of  the  bornite  penetrate  the 
chaloopyrite,  but  in  many  oases  if  the  forms  of  the  two  miner- 
als wero  interchanged,  their  relations  to  each  other  would  be 
little  altered*  The  term  mutual  boundary  has  been  found  use- 
ful in  referring  to  the  line  of  contact  of  minerals  in  this 
association  (Fig.67).  There  is  evidence,  however,  that  the 
ohalcopyrite  slightly  preceded  the  bornite,  as  inclusions  of 
the  former  are  commoner  in  the  latter  than  the  reverse,  even 
where  the  two  sulphides  occur  in  equal  amounts,  and  in  a  few 
oases  the  chalcopyrite  shows  slight  corrosion  by  the  bornite* 
Nevertheless,  for  the  greater  part  of  their  periods  of  forma- 


jfcns 


115 


tion,  they  were  probably  contemporaneous. 

Aa  the  proportion  of  the  chlorite  and  epidote  grows,  in- 
dicating increased  departure  from  magmatio  conditions,  the 
ores  become  more  and  more  ragged  and  irregular,  and  in  the  thin 
section  are  easily  distinguished  by  their  form  along  from  the 
more  regular  magnetite  grains.  They  seem  also  to  hare  exerted 
a  greater  corrosive  action  on  the  rock  minerals,  the  amphibole 
especially  suffering,  although  the  other  constituents  are  not 
spared. 

The  enargite  occurs  sparingly  with  the  other  sulphides, 
and  is  apparently  of  about  the  same  age.  Tetrahedrite  is  per- 
haps a  little  commoner  and  its  presence  may  account  for  the 
slight  silver  value  of  the  ore.  It  occurs  in  the  same  rela- 
tionship to  the  bornite  and  ohaloopyrite  as  the  enargite.  A 
few  minute  grains  ooouring  in  hornblende  were  identified  as 
galena. 

Calcite  is  abundant  in  most  of  the  specimens.  It  clear- 
ly replaces  amphibole  and  feldspar,  and  is  associated  with 
the  epidote  in  an  intimate  way,  veinlets  of  each  cutting  the 
other.  The  oaloite  had  a  longer  range,  however,  extending 
into  the  period  of  later  hydrothermal  conditions.  Rogers 
mentions  finding  calcite  as  a  late  magmatic  mineral.  In  our 

thin  sections,  the  calcite  is  clearly  a  replacement  of  earli- 

(Fig.  61) 
er  minerals,  although  in  certain  areas  the  replacement  is  so 

A 

complete  that,  if  evidence  from  other  portions  of  the  slide 
had  not  revealed  its  true  character,  it  might  have  been  mis- 


116 


taken  for  an  earlier  constituent  of  the  rook.  In  no  case, 
however,  was  there  the  least  doubt  of  its  distinctly  later 
character* 

LATE  HYDROTHISRitAL  PERIOD. 

Development  _of  Zeolites  and  Carbonates*  -  One  of  the  most  in- 
teresting phases  of  the  rock  alteration  is  the  development  with 
the  carbonates  of  a  series  of  zeolites,  as  a  late  hydrothermal 
product*  The  following  members  of  the  group  were  definitely 
determined,  by  means  both  of  thin  sections  and  of  index  liquids: 
soolesite,  analoite,  heulandite,  natrolite,  and  laumontite* 
Thin  radial  aggregates  of  thomsonite  and  small  patches  probably 
of  chabazite  and  phillipsite  were  identified  in  thin  sections 
only,  but  the  minerals  are  not  present  in  sufficient  quantities 
to  permit  of  their  determination  with  absolute  certainty.  A 
little  prehnite,  found  in  only  one  section,  is  probably  a  pro- 
duct of  the  same  conditions  as  the  zeolites*  The  minerals  of 
the  zeolite  group  are  fairly  widespread,  occurring  in  over  one 
third  of  the  specimens  studied* 

In  one  specimen,  soolesite  has  completely  replaced  the 
plagioolase,  whose  former  presence  is  indicated  only  by  the 
zeolite-filled  molds  of  magnetite,  which  had  shaped  themselves 
about  the  original  feldspar  laths*  A  surprising  feature  is 
the  presence  of  unaltered  hypersthene,  bronzite  and  diopside 
in  this  greatly  altered  rook;  having  apparently  escaped  the 
alteration  to  amphibole  in  the  pneumatolytio  period,  the  pyrox- 


117 


enes  remained  resistant  to  the  zeolitizing  conditions.  The 
sooleaite  also  forms  a  veinlet  half  an  inch  wide  of  finely 
radial,  white  fibers,  and  in  it  are  a  few  grains  of  bornite, 
which  show  the  feeble  extension  of  the  bornite  forming  condi- 
tions into  the  period  of  the  zeolites. 

The  commonest  type  of  zeolitio  alteration  is  the  replace- 
ment of  feldspar  by  heulandite,  and  in  about  half  of  the  cases, 
the  subsequent  replacement  of  the  heulandite  by  natrolite. 
The  analcite  in  the  rocks  was  observed  by  Rogers,  who  regarded 
it  as  a  primary  constituent.  In  one  or  two  instances,  it  oc- 
curs as  shapeless  grains,  which  give  no  clue  to  their  origin, 
but  in  other  cases  it  seems  unquestionably  to  be  a  replacement 
of  feldspar,  and  its  association  with  the  other  zeolites  con- 
firms this  view.  Caloite  or  siderite  is  usually  present,  re- 
placing even  the  latest  zeolites,  although  occasionally  they 
are  themselves  out  by  veinlets  of  natrolite* 

Our  evidence  is  not  sufficient  to  permit  the  entire  list 
of  zeolites  observed  to  be  grouped  in  the  precise  order  of 
their  formation,  but  the  analcite  and  heulandite  are  clearly 
earliest,  with  natrolite,  laumontite,  and  thomsonite  forming 
a  later  group.  The  relative  positions  of  the  others  are  in 
doubt.  The  carbonates  are  persistent,  and  in  many  places  the 
completeness  with  which  they  replace  the  preceding  minerals 
has  greatly  obscured  the  evidence  of  the  earlier  relationships, 
Siderite  is  less  common  than  calcite,  but  it  is  frequently 
found  as  a  border  about  caleite  grains,  being  readily  distin- 


.iof«r.  t 


IIS 


guished  by  its  much  higher  relief,  better  crystal  form,  and 
slight  iron  staining  where  exposed  to  oxidation.  It  is  not 
established  with  certainty,  however,  that  the  siderite  is  & 
member  of  this  mineral  group  formed  under  late  hydrothermal 
conditions;  the  possibility  that  it  is  a  product  of  descend 
ing  oxidizing  solutions  will  be  considered  later. 

PERIOD  OP  OXIDATIOH  A3D  S 


Between  the  period  of  the  formation  of  the  zeolites 
and  the  exposure  of  the  ore  to  the  influence  of  oxidation, 
the  ore-bearing  rock  was  again  broken  by  series  of  roughly 
parallel  cracks,  which  became  filled  with  fine  veinlets  of 
siderite  and  calcite.  The  breaks  are  abundant  in  certain 
zones,  and  almost  lacking  in  others*  They  show  no  relation 
to  the  earlier  shearing  and  straining  of  the  rock*  All 
minerals  and  structures  previously  described  are  broken  by 
them,  except  that  they  show  a  marked  avoidance  of  bornite 
grains  which  lie  in  their  paths.  Occasional  carbonate  vein- 
lets  out  through  the  bornite,  but  far  more  commonly  they  axe 
diverted,  and  pass  around  the  border  of  the  sulphide  grains* 
On  the  other  hand,  the  magnetite  and  ilmenite-hematite  inter- 
growths  are  greatly  cracked,  as  if  they  were  more  brittle, 
and  are  penetrated  by  the  veinlets  wherever  these  are  notably 
developed.  The  earlier  shearing  and  recrystallization  of  the 
rook,  and  the  later  fractures  occupied  by  voinlets  were  of 
great  importance  in  facilitating  the  downward  movement  of  sur- 


too  e;r«».f 


119 


face  solutions  by  which  oxidation  and  descending  secondary 
enrichment  were  accomplished. 

?one  of  Complete  Oxidation.  -  The  outcrop  of  the  iingels 

deposit,  which  lies  at  an  elevation  of  about  5,500  feet,  is 

(Figs.  7  and  8). 
wholly  inconspicuous. A  A  mantle  of  granular  soil  conceals 

the  rook  except  at  a  few  places  where  slightly  copper-stained 
croppings  are  exposed  but  do  not  project  noticeably  above 
their  surroundings.  The  orebody  is  capped  by  a  moderate  but 
Irregular  thickness  of  oxidized  and  leached  rook.  Plainly 
thio  material  once  contained  sulphides  for  it  is  stained  by 
limonite.  malachite  and  ohrysocolla,  and  is  out  by  banded 
seams  and  small  veins  of  these  minerals.   In  these  vainleta, 
the  silicate  aeems  to  have  been  the  last  to  form,  frequently 
breaking  aotoas  the  carbonate  bands,  and  building  out  as 
knobby,  botryoidal  forms  into  open  spaces  presumably  formed 
by  removal  of  the  sulphides. 

Zone  £f  Sulphide  Enrichment.  -  According  to  Mr. Juessen, 
who  was  in  charge  when  the  upper  parts  of  the  orebodies  were 
developed,  the  zone  of  complete  oxidation  changes  irregularly 
into  a  zone  of  mixed  carbonate  and  chalcocite  ore,  rich  por- 
tions of  which  were  mined,  but  caving  has  made  these  upper- 
most stopes  now  inaccessible.   This  was  succeeded  immedi- 
ately below  by  a  much  more  definite  and  regular  zone  of  chal- 
cooite  ore,  of  which  much  the  largest  and  richest  portions 
were  discovered  subsequent  to  the  examination  made  by  Mr. Tur- 
ner.  Ore  of  this  kind  was  found  in  a  locus  averaging  about 


120 


twenty-five  feet  thick,  and  dipping  approximately  parallel 
to  the  surface,  which  slopes  rather  gently  to  the  southwest. 
A  very  considerable  tonnage  of  such  chaloocite  ore  was  mined 
out,  with  values  up  to  16  per  cent,  copper,  and  an  average 
grade  materially  higher  than  that  of  the  ore  at  greater  depth. 
In  general,  this  ore  gradually  fingered  out  at  depths  of  about 
90  to  130  feet  below  the  outcrop  into  ore  consisting  essen- 
tially of  bornite,  but  occasional  stringers  and  bunches  of  chal- 
oocite ore  extend  considerably  deeper. 

In  nearly  all  the  chips  studied,  chalcocite  was  found  in 
varying  amounts,  commonly  as  a  replacement  of  bornite  and  to 
a  very  small  extent  a  replacement  of  chalcopyrite.  Its  moat 
striking  development  is  in  connection  with  the  veinlets  of 
carbonate  previously  discussed,  and  this  relationship  persists 
to  the  deepest  v/orklngs  of  the  mine  (June,  1916),  a  winze  Iii5 
feet  below  the  level  of  Ho.  4  Tunnel  or  about  375  feet  from 
the  surface.   In  many  cases  chalcooite  occurs  only  where  th« 
bornite  is  broken  or  bounded  by  such  veinlets.   The  abrupt 
manner  in  which  laths  of  sericito  are  cut  by  the  voinlets  with 
which  tho  chalcocite  is  associated,  leaves  no  doubt  of  the  inde- 
pendence of  orisrin  of  the  ohaloocite  from  the  sericite.   Vein- 
lets  of  chalcooito  are  frequently  observed  to  take  advantage  of 
the  presence  of  included  laths  of  chlorite  or  sericite  along 
their  course,  breaking  from  one  to  another,  while  other  laths 
nearby,  but  not  intercepted  by  the  veinlets,  may  be  sharply 
bounded  by  the  bornite  without  a  trace  of  chalcocito.  7iftiere 


121 


the  bornite  occurs  in  small  ragged  slivers  in  aggregates  of 
epi-lote,  chlorite  or  sericite,  the  enrichment  is  moat  com- 
plete, due  both  to  the  greater  permeability  of  the  rock  at 
such  places,  and  to  the  relatively  greater  surface  of  bornite 
exposed  by  the  finer  grains.   Chalcocite  ia  commonly  wall 
developed  alao  where  the  bornite  is  penetrated  by  laths  of 
chlorite  or  aericito,  although  occasionally  no  enrichment  is 
observed  in  connection  with  these  minerals.   Our  studies  of 
chalcocite  formation  in  many  districts  indicate  that  enrich- 
.ent  favors  the  contacts  between  primary  sulphides  and  gangue 
minerals  (Pig.  74  ),  because  of  the  greater  local  permeability 
along  these  junctions,  and  that  micaceous  gangue  minerals,  aa 
aericite  or  chlorite,  notably  promote  both  local  and  general 

enrichment,  as  Beeson  has  ao  clearly  shown  in  the  case  of 

1 
Bingham. 

In  addition  to  the  replacement  of  bornite  along  irregu- 
lar ve inlets  and  grain  boundaries,  the  ohalcocite  alao  re- 
places that  mineral  along  crystallographio  planes,  '.yhere  this 
type  of  replacement  is  well  developed,  the  polished  surface 
reveals  a  strikingly  regular  pattern,  consisting  of  three  in- 
tersecting systems  of  narrow  strips  of  chalcocite,  separated 
by  triangular  and  rhombic  residues  of  the  bornite;  the  well- 
known  relationship  which  is  termed  the  lattice  structure. 

1.  Beeson,  J.  J.  T.  j..l.  I.  .,  Vol.54,   pp.  402-441.  1915- 


. 


I 


122 


Associations  of  bornite  and  ohaloocite  in  graphic  struc- 
tures are  present  sparingly  throughout  the  deposit  aa  now  de- 
veloped* except  where  the  bornite  has  been  completely  con- 
verted into  ohalcocite  (Pigs. 69, 70, 75  and79>  These  graphic 
structures  at  3ngels  occur  in  the  rafdst  of  bornite  masses 
away  from  fractures  and  grain  boundaries,  and  they  also  oc- 
cur in  the  marginal  portions  of  bornite  areas  as  well  as  ad- 
jacent to  veinlets  of  chalcocite  from  which  the  graphic  ohal- 
oocite  is  not  separ  ,ted  by  any  visible  demarcation.  The  ques- 
tion of  the  origin  and  significance  of  these  graphic  struc- 
tures, which  is  one  of  considerable  complexity,  will  be  given 
attention  on  a  subsequent  page. 

Covellite  is  occasionally  produced  in  subordinate  amounts 
by  partial  marginal  replacement  of  bornite  and  chalcopyrite. 
In  a  few  places,  either  in  ohalcooite  near  bornite  or  in  the 
bornite  itself.it  forms  fine  lines  on  the  polished  surface, 
which  show  the  same  structural  control  by  the  bornite  as  has 
beon  described  for  the  chalcocite  and  bornite  in  the  lattice 
structure.   Examples  are  clearly  illustraded  by  sketches  and 
photographs  in  Turner  and  Roger's  paper. 

Secondary  chalcopyrite  is  found  to  a  relatively  small  ex- 
tent.  It  ia  commonly  asaociated  with  chalcocito,  but  appears 
to  prefer  small  cracks  or  nooks  in  the  bornite  somewhat  removed 
from  the  paths  of  strongest  ohalcocite  development.  The  re- 
placement of  bornite  by  chalcocite  involves  the  removal  of 
iron  as  well  as  the  addition  of  copper,  and  the  small  isolated 


veinlets  or  spines  of  ohalcopyrite  probably  represent  local 
oonoentration  of  iron—  small  eddies,  as  it  were,  in  the 
main  stream  of  enrichment  —  where  conditions  of  equilibrium 
were  temporarily  reversed,  and  the  iron-rich  sulphide  formed. 
The  ohaloopyrite  frequently  penetrates  the  bornite  along  def- 
inite structural  lines,  producing  in  some  cases  a  lattioe  pat- 
tern similar  to  that  described  for  chaloocite  and  for  covel- 
lite  in  bornite  (Pig.  77) •   Inasmuch  as  various  stages  have 
been  observed  from  a  few  isolated  spines  penetrating  the  born- 
ite from  the  edge  of  a  ve inlet  of  ohaloocite  due  to  downward 
enrichment  to  the  well-developed  lattice-work,  there  oan  be 
no  doubt  that  these  ohalcopyrite  spines  are  of  the  same  origin 
as  the  ohalcocite.   In  the  specimen  which  shows  the  moat 
striking  development  of  secondary  ohalcopyrite,  the  high  con- 
centration of  iron  and  the  influence  of  surficial  conditions 
are  indicated  by  the  development  of  a  strong  seam  of  limonite, 
to  which  the  distribution  of  the  chalcopyrite  shows  a  defin- 
ite relationship. 

DISCUSSION. 

Genetic  Classification  of  the  Deposit* 

The  grouping  of  the  minerals  (p.!23a)is  merely  a  summary  of 
the  interpretation  of  the  genesis  of  the  deposit  presented  on 
the  preceding  pages,  and  so  needs  little  further  discussion. 
The  magmatio  group  includes  only  the  minerals  which  crystal- 
lized directly  from  the  differentiated  magma.  *.s  the  separa- 


123 -a 


DIAGRAM  0?  MINERAL  SEQUENCE. 

^_             Period 

Mineral 

Mag- 
natio 

Pneunat 
olytic 

••  Intense 
Hydro- 
thermal 

Late 
Hydro- 
ther- 
gal 

Ox  Ida" 
tion  & 
enrioh 

merit  * 

1.   Zircon 
2.   Hutlls 
3.   nronzite 
4.  Hypers  thene 
5.  Dlopside 
o.   Labradorite 
7.   Biotite 
ff.   Apatite 
y.   Magnetite 
10.   Spinel 



— 



— 

— 

11.  Hornblende 
12.   Actinolite 
13.   Albite 
I1*-.   Oli^oclase 
15-   Orthoclase 
16.  i'lcrocline 
17.   Iliaenlte 
1#.   Hematite 
19.   Tourmaline 
HO.  Titanite 
(  as  leucoxene  ) 
HI.   Quartz 

—  _^_ 



HH.  Chlorite 
H3.   Sericite 
HM-.  Epidote 
25.   -ioisite 
26.  Chalcopyrite 
27.  Bornite 
2flf.  Enargite 
H^.   Tetrahedrite 
30.   Galena 
?1.  Sphalerite 



^2.  Anilcite 
33-  Heulandite 
3^-.  Scolesite 
35.  Prehenite 
36.   Thonusonite 
37.   dhabazite  (  ?  ) 
3*.  Phillipsite  (  ?  ) 
39.  Natrolite 
'4-0.   Lausionite 
*H.  Calcite 

^^-^— 

7 



^•2.   Siderite 
U-3.   Covellite 
U-M-.   Chalcocite 
4-5.  Liala'-hite 
^.  Llnonite 
^7.   Chncaooolla 

-^ 

— 

sequence  given  under  tho  j^*^  „*  WA 
tion  ai;l  enrichment  are  intended  to  show  trv.  progressive  ar- 
rival of  various  influences  at  any  given  horizon.   In  reality, 
probably  all  tho  minerals  of  the  period  have  been  forming,  at 
one  part  or  another  of  the  deposit,  from  the  beginning  of  oxida- 


tlon  of  the  normal  rook-forming  constituents  became  more  and 
more  complete,  the  concentration  of  volatile  constituents  in 
the  residual  magmatic  material  steadily  increased,  and  condi- 
tions  of  normal  magmatio  crystallization  gradually  changed  to 
those  in  which  pneumatolytic  processes  played  the  dominant 
part.   The  widespread  alteration  of  pyroxene  to  amphibole, 
the  formation  of  tourmaline,  the  small  lenticular  segregations 
and  dikes  of  pegmatitio  nature  are  attributed  to  this  phase. 
Following  this  type  of  alteration,  changes  of  distinctly  hy- 
drothormal  character  were  imposed  upon  the  rock,  differenti- 
ated from  tho  preceding  pneumatolytic  conditions  by  the  gradu- 
ally increasing  abundance  of  chlorite  and  epidote  in  particu- 
lar.  The  closing  relatively  mild  phases  of  hydrothermal  ac- 
tion are  characterized  by  the  series  of  zeolites  and  by  car- 
bonates. 

The  amphibole  and  the  major  amount  of  the  iron  oxides 
associated  with  it  are  to  be  regarded  as  products  of  pneuma- 
tolysis,  both  on  account  of  their  position  in  the  general  min- 
eral sequence,  and  from  the  abundance  of  apatite  and  occa- 
sional, inclusions  of  tourmaline  associated  with  them.   The 
period  of  magnetite  formation  may  have  continued,  however, 
into  the  early  intense  phases  of  the  hydrothermal  period. 

The  principal  coppor  ores,  ohalcopyrite  and  bornite, 
commenced  to  form  under  pneumatolytic  conditions,  as  indi- 
cated by  their  occurrence  in  the  pegmatite,  but  they  attained 


*»  *»  y  »>  *  * 


:•     :-i 


125 


their  maximum  development  when  accompanied  by  minerals  of  ac- 
cepted hydrothermal  origin,  and  even  continued  in  a  feeble  way 
into  the  period  characterized  by  the  zeolites.   The  percent- 
age of  the  ores  connected  with  the  pegmatites,  or  as  occasion- 
al amall  inclusiona  in  the  magnetite,  is  small,  and  there 
seems  to  be  good  evidence  that  the  larger  part  of  the  sulphides 
the  part  which  makes  the  deposit  a  commercial  orebody  -  is  of 
hydrothermal  origin. 

Turner  and  Rogers  regard  the  ore  as  a  magmatic  segregation, 
From  field  evidence.  Turner  considers  the  sulphides  to  be  the 
final  products  of  the  crystallization  of  the  magma,  analagous 
to  the  quartz  in  a  granite,  and  to  have  been  formed  before  the 
development  of  the  pegmatites,  ^s  a  matter  of  fact,  such  a 
view  is  justified  by  the  general  appearance  of  much  of  the  ore 
and  by  many  of  the  broad  field  relations,  though  it  is  contra- 
dicted by  certain  megascopic  features,  subordinate  in  distribu- 
tion but  significant  in  character,  such  as  the  schistose  struc- 
ture of  some  of  the  ore-bearing  rock,  and  the  presence  of  sul- 
phides both  along  fractures,  and  in  seams  of  pegmatite  and 
rarely  of  zeolites.  Rogers  concludes,  from  microscopic  examin- 
ation of  Turner's  specimens,  that  the  ores  are  products  of  gaa- 
eous  solutions,  rioh  in  mineraliaers,  and  evidently  assigns  to 
them,  a  later  origin  than  did  Turner.  Although  pointing  out 
that  deLaunay  groups  deposits  of  such  origin  under  the  heading 


126 


Gritea  de  depart  imme'diat ,  Rogera  retains  the  term  "magmatic 
segregation,"  because  of  the  close  analogy  he  believes  to  exist 
between  the  orea  at  Bngols  and  those  at  Sudbury,  Insizwa  and 
other  deposits  that  are  commonly  regarded  as  magmatic  segrega- 
tions. 

Che  phase  in  the  geologic  history  of  tho  angels  deposit 
which  seems  to  us  to  correspond  most  closely  to  the  conditions 
of  origin  postulated  by  deLaunay  for  the  deposits  classified  by 
him  as  Sites  de  depart  imm^diat ,  is  what  we  have  termed  the 
pneumatolytic  period.  The  iron  oxides  at  Engels  and  a  small 
proportion  of  the  bornite  and  chalcopyrite  were  formed  at  this 
stage*  They  might  possibly  be  classified  as  magmatic  segrega- 
tions provided  the  term  may  properly  be  expanded  to  such  an  ex- 
tent* But  the  close  association  of  the  greatest  part  of  the 
primary  copper  sulphides  with  hydrothermal  minerals  is  distinct- 
ly different  from  the  conditions  as  described  for  Sudbury  or 
other  deposits  regarded  as  raagmatic,  and  unquestionably  classi- 
fies the  Engels  orebody  as  a  hydrothermal  deposit. 

ORIQIE  OP  TH3  CHALCOCITE. 

Ghaloocite  Clearly  of_  Replacement  Origin.  -  Rogers  first 
attributed  the  formation  of  chalcocite  at  Sngels  entirely  to 
the  agency  of  ascending  alkaline  waters  and  considered  it  a 
hydrothermal  effect.  In  his  later  publication  he  recognizes 
that  chalcooite  due  to  descending  waters  is  present,  but  main- 
tains that  "the  evidence  in  favor  of  'upward  enrichment1  is 
even  stronger  than  before."  The  absence  of  kaolin,  the  occur- 


5" 


i  »e 

•  .JbolTW  oi*ilJo*amirr 


tw 


erf*  r 


:o  riolJfiDrcoi 

".3 


127 


rence  of  chalcocite  in  massive  ore-bearing  rocks,  and  the  re- 
lation of  the  chalcocite  to  the  sericite  and  chlorite  are  points 
he  has  urged  to  support  the  thesis  of  "upward  secondary  enrich- 
ment." They  may  be  considered  in  order. 

The  absence  of  kaolin  was  advanced  in  Rogers fs  first 
paper  as  a  proof  that  the  chalcocite  is  not  the  product  of  de- 
scending solutions.  Confirming  his  observations,  we  find  that  it 
is  not  a  common  product  at  iilngels,  although  small  amounts  were 
found  in  a  few  oases.  The  formation  of  kaolin  depends  primari- 
ly on  the  presence  of  sulphuric  acid,  which  in  turn  is  largely 
derived  from  pyrite.  As  no  pyrite  has  been  observed  at  Engels, 
the  lack  of  kaolin  with  the  oxidized  products  is  not  surprising. 
The  solutions  descending  from  the  oxidized  zone  are  probably  at 
moot  only  mildly  acid,  as  they  do  not  noticeably  attack  the  car- 
bonate present  in  the  rook  or  even  in  the  veinlets  with  which 
chaloocite  is  associated,  but  there  is  no  reason  to  believe 
that  such  solutions  are  incapable  of  producing  chalcocite  en- 
richment. Our  observations  in  many  districts  indicate  that 
the  enrichment  of  bornite  to  chalcocite  by  descending  waters 
is  accomplished  so  readily,  and  with  the  production  of  so 
little  sulphuric  acid,  that  it  may  take  place  without  the  ap- 
preciable development  of  kaolin. 


1.  The  generalizations  in  this  paragraph  regarding  kaolin 
have  been  drawn  in  large  part  from  investigations  being  carried 
on  by  our  associate,  Mward  H.  Perry. 

2.  Zies,  E.  G.  Allen,  E.  I'.,  and  Herwin,  H.  £!.,  "Some  Reac- 
tions Involved  in  Secondary  copper  Sulphide  Enrichment,"  Boon. 
Geol.,  Vol.  11,  p.  500,  1916. 


12S 


Chalcocite  la  frequently  developed  from  bornite  at 
Engels  in  hard,  dense  rooks  beyond  the  limits  of  the  oxidized 
materials,  but  notwithstanding  the  great  ease  with  which  born- 
ite ia  enriched,  this  transformation  is  even  here  dependent 
upon  the  degree  to  which  chlorite  and  sericite,  or  microscopic 
fractures  have  materially  augmented  the  normal  permeability  of 
the  ore  due  chiefly  to  minute  channelways  along  gangue  bounda- 
ries* 

In  his  first  paper,  Rogers  assigned  an  earlier  age  to  the 
chlorite  than  to  the  bornite  and  chalcopyrite,  or  the  chalcocito, 
but  stated  that  the  sericite  is  younger  than  the  bornite  and 
chalcopyrite,  and  that  it  developed  simultaneously  and  in  inti- 
mate genetic  association  with  the  ohaloocite*  In  hia  later  ar- 
ticle, the  chalcocite  which  he  attributes  to  ascending  solu- 
tions is  considered  older  than  both  the  chlorite  and  the  seri- 
cite, while  the  chalcocite  which  he  ascribes  to  descending  in- 
fluences ia  regarded  as  younger  than  these  gangue  minerals. 
Both  kinds  of  chalcocite,  he  adds,  were  formed  independent  of 
the  process  of  sericitization,  and  he  now  accepts  Beeson's  ex- 
planation of  greater  permeability  to  account  for  the  close  po- 
sitional relation  observed  between  the  sericite  and  chlorite 
and  some  of  the  chalcooite. 

As  we  interpret  the  evidence,  the  chlorite  and  sericite 
laths  are  to  be  regarded  as  inclusions  in  the  bornite-ohalcopy- 
rite  matrix,  yet  formed  at  practically  the  same  time  as  their 
host,  and  perhaps  developed  with  more  perfect  outline  chiefly 


129 


because  of  their  greater  power  of  crystallization.  We  find  no 
satisfactory  foundation  for  Rogers 's  view  that  the  latha  of 
aericite  and  chlorite  have  been  formed  by  later  replacement  of 
the  bornite  and  ohalcopyrite.  We  are  convinced  that  this  is 
not  true  for  the  chlorite.  As  for  the  sericite,  however,  the 
evidences  (pp. 112 and  113)  controverting  his  interpretation  are 
not  so  abundant  or  so  forceful,  and  our  dissent  from  it  arises 
as  much  from  its  incompatibility  with  the  tendencies  of  se- 
quence in  sulphide  ores  in  general  and  from  the  notable  simi- 
larity of  habit  and  occurrence  between  the  sericite  and  the 
chlorite,  as  from  any  compelling  evidence  against  it  in  this 
particular  case. 

i"ith  respect  to  the  chalcocite,  except  for  the  compara- 
tively small  amounts  in  the  graphic  areas  which  will  be  dis- 
cussed later,  the  evidence  is  conclusive  that  it  is  later 
than  the  serioite  or  the  chlorite.  The  greater  general  de- 
velopment of  chalcocite  in  those  parts  of  the  ore  containing 
abundant  sericite  and  chlorite  is  readily  explained  as  a  re- 
sult of  the  greater  permeability  caused  by  the  presence  of 
these  micaceous  minerals,  as  already  noted* 

Between  the  chalcocite  in  the  lattice  structure   on 
the  one  hand,  which  Rogers  regards  as  due  to  ascending  altera- 
tion, and  on  the  other  hand  the  chalcocite  of  admittedly  de- 
scending origin  which  constitutes  more  or  less  regular  rims 


1.   See  Fig.  8  of  Rogers 's  article  in  Kcon.  Geol.,  Vol.  11, 
p.  582,  1916. 


1 

about  laths  of  chlo'rite  and  serioite   and  grains  of  other 

&  a 

gangue .minerals   or  forms  veinlets   with  or  without  accom- 
panying siderite  or  limonite,  we  find  it  impossible  to  dravr 
any  important  distinctions  as  to  gcmesis,  and  we  aro  forced 
to  conclude  that  both  kinds  of  chalcocito  were  formed  in 
essentially  the  same  manner  and  "by  essentially  the  same  means. 

Examination  of  Rogers 's  Fig.  2,  to  which  reference  has 
been  made,  and  of  our  Jig.  74-  reveals  the  presence  of  numerous 
narrow  tongues  or  spines  of  chalcocite  extending  from  the  chal- 
cocite  rims  into  the  bornite  cores  and  arranged  in  several  sys- 
tems of  directions.  This  feature,  which  is  very  common  at 
Engels,  we  have  found  in  many  districts  to  be  simply  an  early 
stage  of  the  development  of  the  lattice  structure.  Moreover, 
this  formation  of  lattice  chalcocite  in  bornite,  the  develop- 
ment of  narrow  gashes  or  spines  of  chalcopyrite,  with  or  with- 
out ohalcocite  or  covellite  (Pig.  J8  )  in  bornite  near  where  it 
is  being  altered  to  chalcocite,  and  the  similar  production  of 
either  felted  aggregates  or  sharp  plates  of  covellite  along 
boraite-chulcocite  boundaries  are  features  very  common  in  con- 
nection with  the  alteration  of  bornite  to  chalcocite  by  the 
normal  process  of  downward  enrichment,  as  is  exhibited  in  such 
unquestionable  examples  as  Bisbee,  Uorenci,  Globe,  A jo,  Bing- 
ham,  and  Ely.  Che  last  two  of  these  features,  furthermore, 

1.  See  Fig.  5  of  same  paper  by  Rogers. 

2.  See  Fig.  2  of  Rogers 's  paper  and  Fig.  J4-  of  this  paper. 

3.  See  Fig.  73  of  this  paper. 


131 


have  been  produced  artificially  in  the  Geophysical  Laboratory 
by  the  action  of  solutions  containing  copper  sulphate  and  sul- 
phuric acid  on  bornite. 

Added  to  these  evidences  favoring  a  descending  origin  for 
the  chalcocite  are  several  others .  The  chalcocite,  though  de- 
veloped in  part  by  replacement  of  chaloopyrite,  is  yielded  main- 
ly by  bornite;  in  other  words  the  bornite  is  much  more  easily 
enriched  than  the  chalcopyrite,  This  is  entirely  in  accord  with 
repeated  observations  in  examples  of  unquestioned  downward  en- 
.richment  and  with  the  results  of  experiments  in  artificial  en- 
richment*  In  this  connection  it  may  be  noted  that  in  the  con- 
version of  bornite  to  chalcocite,  only  a  relatively  small  addi- 
tion of  copper  is  required  to  produce  a  result  of  important 

£ 

magnitude,  for  the  ono  mineral  contains  63.33  per  cent,  copper 
or  almost  4/5  as  much  as  the  other,  viz.,  79.86  per  cent.  .?ur- 
thermore,  the  oxidation  and  leaching  of  a  zone  of  solid  born- 
ite ore  would  liberate  and  make  available  for  enrichment  at 
greater  depth  more  than  twice  as  much  copper  as  would  a  solid 
chalcopyrite  ore  of  equal  thickness.  Thus,  as  regards  quan- 
tity of  chalcocite  produced,  bornite  is  doubly  at  an  advantage 
over  any  of  the  sulphides  poorer  in  copper. 

In  the  particular  case  of  the  Engels  deposit,  all  the 


1.  Sies,  Allen,  and  Menvin,  Loo.  cit.,  pp.  498-499. 
2*  Allen,  &.  T.,  "She  Composition  of  Natural  Bornite,"  Am. 
Jour.  3oi.,  Vol.  41,  pp.  409-413,  1916. 


132 


chalcocite  thus  far  encountered  could  have  been  produced, 
through  enrichment  of  tho  original  sulphides,  by  a  decidedly 
smaller  amount  of  copper  than  has  plainly  been  removed  from 
the  tipper,  oxidized  zone,  notwithstanding  the  incompleteness 
of  leaching;  and  although  quantitative  data  obviously  are  un- 
available, it  is  not  improbable  that  this  source  was  suffi- 
cient, even  thoxigh  the  enrichment  process  may  have  been  waste- 
ful of  copper.  In  any  event,  it  is  evident  that  the  Engels 
deposit  once  continued  above  the  present  surface  and,  as  ap- 
pears later,  topogra  lie  conditions  were  favorable  for  enrich- 
ment during  the  erosion  that  cut  down  to  the  present  outcrop* 
The  copper  furnished  by  leaching  of  a  relatively  small  thick- 
ness of  the  ore  now  eroded  away,  added  to  that  removed  from 
the  present  leached  zone,  would  unquestionably  suffice  to  ac- 
count both  for  any  reasonable  degree  of  waste  in  the  enrichment 
process  and  for  an  extension  of  enriching  chaloocite  in  gradu- 
ally decreasing  abundance  to  a  very  considerable  depth  below 
the  horizon  to  which  the  mine  workings  have  yet  been  carried. 

As  a  matter  of  fact,  if  all  the  ehalcocite  at  Kngels  be 
regarded  as  due  to  descending  waters,  the  amount  of  enrichment 
la  moderate  or  slight  as  compared  with  that  in  other  bornite- 
rich  deposits  of  comparable  physiographic  history. 

Finally  in  the  upper  parts  of  the  mine,  there  are  asso- 
ciated with  the  chalcocite,  veinlets  of  liraonite  and  of  sider- 
ite  (with  perhaps  a  little  quartz),  while  at  greater  depth, 
tho  limonite  disappears  but  the  siderite  persists.  These  pro- 


dticta  thus  disposed  are  what  might  be  expected  to  result 
from  iron-bearing  solutions  at  different  distances  from  the 
source  of  oxygon  supply,  i.  e.,  the  surface,  and  of  course 
the  fixation  of  iron  in  a  partly  or  completely  oxidized  con- 
dition in  the  circulation  channel-ways  at  either  horizon  ac- 
corda  with  the  idea  of  replacement  of  a  copper-iron  sulphide 

by  a  copper  sulphide  under  conditions  of  descending,  oxidi- 

1 
zing   enrichment.  Siderite,  associated  v/ith  a  little  chal- 

2 
cocite,  has  recently  been  described  at  Bisbee  aa  the  deepest 

prodiict  of  oxidising  influences.   In  the  Bisbee  occurrence, 
the  siderite  is  largely  a  replacement  of  oaloite  (limestone). 
At  Engels,  the  aiderite  in  the  veinlets  is  pretty  certainly 
a  product  of  surface  influences;  but  as  to  the  siderite  that 
appears  to  replace  areas  of  caloite  in  some  of  the  aeolite- 
bearing  rocks,  we  are  somewhat  in  doubt  whether  it  likewise 
was  formed  by  meteoric  waters  or  is  a  final  product  of  hy- 
drothermal  action*  Whatever  its  origin,  the  total  amount  of 
sidorite  at  Sngels  is  insignificant. 

Covellite,  which  at  Engels  is  wholly  subordinate  in 
qitantity  to  chalcocite,  is  a  replacement  of  earlier  sulphides, 
and,  like  the  replacement  chalcocite,  is  competently  explained 


1.  See  Spencer,  A.  C,,  Jour.  Washington  Acad.  8ci«,  Vol. 

3.  pp.  70-75,  1915.  Also  Qraton  and  Murdoch,  loc.  cit.,  p.  65. 

2.  Bonillas,  Y.  S.,  Tenney,  J.  B.,  and  Feuchere,  Leon, 
"Geology  of  the  Warren  Mining  District,"  Bull.  A.I.M.E. ,  Sept., 
1916,  pp.  1451  and  1457. 


ori 


on  the  ground  that  it  is  of  superficial  origin.  We  are  un- 
able to  follow  Rogers 'a  argument  that  chlorite  replaces  covel- 
lite,  but  find  that  covellite,  like  chaloocite,inay  replace 
bornite  and  chaloopyrito  along  the  margins  of  chlorite  laths 
and  along  gangue  boundaries  in  general. 

In  short,  we  find  at  lintels  that  all  the  clearly  enriching 
chalcocite  -  that  plainly  formed  by  replacement  .of  earlier, 
leaner  sulphides  -  is  to  be  explained  as  a  result  of  the  action 
of  superficial  solutions,  and  that  there  is  no  need  or  justifi- 
cation for  resorting  to  the  idea  of  "ascending  secondary  enrioh- 
raont . " 

ChalcQcito  in  the  Graphic  Struct  tire,  -  The  discussion  under 
tho  preceding  section  applies  to  the  chalcooite  which  gives 
ready  evidence  of  having  been  derived  by  replacement  of  bornite 
(and  a  little  ohalcopyrite) ,  and  only  when  all  tho  ohalcocite 
of  the  deposit  has  been  mentioned  has  reference  been  intended 
to  the  chalcocite  that  occurs  in  graphic  relation  to  bornite. 
This  graphic  ohalcocite  is  present  in  amount  decidedly  subor- 
dinate to  the  chalcooite  clearly  of  replacement  origin. 

In  OMT  opinion,  the  validity  of  the  hypothesis  of  ascend- 
ing origin  for  part  of  the  3ngels  chalcooite  hinges  on  the 
source  and  character  of  the  ohalcocite  in  the  graphic  structures. 
Concerning  the  origin  of  the  graphic  relations  between  bornite 
and  ohalcocito,  which  have  boon  observed  in  sevoral  districts, 
a  marked  diversity  of  opinion  results  from  the  difficulty  of 
the  problem  and  the  conflicting  nature  of  much  of  the  evidence. 


135 


The  jHPObloia  problem  of  their  origin  however  will  be  con- 
sidered in  detail  on  later  pages,  whore  more  general  evi- 
dence will  be  presented  than  that  offered  by  the  Angela  de- 
posit, alone* 

If  the  replacement  origin  of  the  graphic  ohalcocite 
at  Engela  is  to  be  accepted,  as  urged  by  Rogers,  w»  find  no 
reason  whatever  for  differentiating  it  in  any  essential  re- 
spect as  to  source  and  age  from  the  chaloocite  of  clearly  re- 
placement origin  which  we  believe  is  unquestionably  due  to 
secondary  enrichment  of  the  orthodox  kind.  But  if  the  graphic 
chalcocite  is  a  result  of  descending  enrichment,  we  believe 
it  represents  the  feeblest  effects  of  that  process  and  is  an 
exceedingly  delicate  ;uanifostution  of  bornite  replacement, 
Possible  light  on  this  idea  is  afforded  by  conditions  at  tho 
Superior  deposit, 

THE  SUIPiir.IOR   iJIHJS. 

An  instructive  parallel  to  the  occurrence  of  the  ore 
at  the  2ngels  Mine  is  offered  by  the  neighboring  deposit  known 
as  the  Superior.  The  hydrothermal  character  of  the  ore  is  very 
definitely  shown  in  the  latter,  and  the  idea  that  the  angels 
ora  is  to  a  large  extent  of  pneunatolytic  and  hydrothermal  ori- 
gin, is  greatly  strengthened  by  the  many  points  of  similarity 
between  the  two  deposits. 

The  Superior  orebody  is  situated  about  two  miles  to  the 
southwest  of  the  Angela  Mine,  and  approximately  1,200  feet 


lev 


. 


136 


lower  in  elevation.  The  ore  outcrop  ca  the  steep  eaatern 
side  of  a  canon,  several  hundred  feet  above  the  stream  near 
the  lower  camp  at  the  foot  of  the  aerial  tramway  from  >the 
Engels  mill.  Aside  from  the  diamond  drill  holes,  the  only 
development  on  the  property  in  June,  1916,  was  a  shaft  about 
forty  foot  deep,  but  a  tunnel  was  being  started  lov/er  on  the 
slope  to  intersect  the  ledge.  The  orobody  consists  of  veins 
of  massive  bornite  and  magnetite,  ranging  from  thin  seams  up 
to  a  width  of  six  inches  or  aore.  They  are  spaced  a  foot  or 
so  apart  in  a  fracture  zone  10-12  faet  wide.   The  lode  as  a 
whole  seems  to  have  a  high  dip,  but  the  individual  veins  are 
more  or  less  irregular. 

The  surrounding  rock  is  a  diorite,  and  is  part  of  the 
same  large  plutonio  body  in  which  the  3ngels  ore  occurs.   It 
is  however  lighter  in  color  than  the  rock  near  the  larger 
mine,  and  examination  shows  it  to  be  of  more  acidic  charac- 
ter. The  feldspar  is  andesino  with  rims  of  oligoclase  or 
oligocluse-albite,  and  the  hornblende  is  lass  prominent  than 
in  the  darker  rock.  The  veins  are  accompanied  by  intense 
rock  alteration.  The  development  of  amphibole,  chiefly  ac- 
tinolito,  at  tho  exponae  of  the  feldspar  or  the  original 
hornblondo,  is  tho  earliest  phase  of  the  mineralisation.  Apa- 
tite in  large  crystals  and  titanito  in  numerous  irregular 
grains  are  abundant,  and  were  probably  produced  at  this  time. 


..exe  i 


Quartz  in  varying  amounts  is  common  under  the  microscope, 
but  is  not  prominent  in  the  hand- specimen.  It  probably  com- 
menced to  form  with  the  amphibole.  The  acidic  rims  of  the 
feldspars  may  also  be  products  of  this  early  phase  of  the 
mineralization.  The  feldspar,  some  of  the  quartz  and  the  ara- 
phibole  all  show  the  effects  of  intense  stresses.  The  amphi- 
bole, especially,  is  fractured  and  sheared.  The  bornite  and 
its  associated  minerals  are  clearly  later  and  take  advantage 
of  the  channelways  thus  afforded.  The  formation  of  the  sul- 
phides was  accompanied  by  the  abundant  development  of  small 
laths  of  a  green  mica,  and  of  epidote,  chlorite  and  serioite. 
The  green  mica  is  commonly  a  replacement  of  feldspar  or  of 
amphibole,  and  the  serioite,  the  least  important  from  a  quan- 
titative standpoint,  is  largely  confined  to  the  feldspar.  The 
latest  mineral  of  the  primary  sequence  is  heulandite,  which 
occurs  abundantly  as  a  replacement  of  the  plagioclase. 

Bornite  and  magnetite  are  by  far  the  most  abundant  ore 
minerals*  The  magnetite  appears  as  large  grains  sometimes 
boldly  euhedral,  surrounded  by  a  cement  of  bornite.  The  lat- 
ter also  occurs  as  fine  blebs  or  irregular  veinlets  in  the 
magnetite;  it  is  clearly  of  later  age  and  in  part  a  replace- 
ment of  the  iron  oxide.  The  association  between  the  two  min- 
erals is  more  intimate  than  in  the  Engels  ore,  and  the  mag- 
netite seems  to  have  been  more  generally  attacked  by  the 
bornite  than  is  the  case  in  the  larger  deposit  (Pigs .^5  and 
2he  absence  of  specularite  and  ilmenite  is  noteworthy. 


f«« 


i  • 


;Ii 


19 £ 


r«A*fn   el    J"  ' 


tvy. 

j   eoc 


13* 


Chaloopyrite  is  less  plentiful  than  at  Engels.  A  few  small 
blebs  of  galena  were  observed. 

Chalcooite  Is  present  in  subordinate  amount  intimately 
associated  in  all  oases  with  the  bornite.  In  small  part,  it 
occurs  as  veinlets  and  tongues  which  are  clearly  replacements. 
In  a  few  oases,  imperfect  lattice  structures  are  developed. 
Chiefly,  however,  it  is  in  graphic  areas  or  in  irregular  patch- 
es with  mutual  boundaries,  and  in  these  forms,  it  commonly 
shows  a  complete  independence  of  the  contacts  between  the  born- 
ite and  magnetite  (Pigs.  86,  88)  •  Where  this  chalcocite  inter- 
sects laths  of  chlorite  included  in  the  bornite,  the  line  of 
contact  is  in  many  cases  likewise  ignored  {Fig.S6  ).  In  some 
oases,  however,  no  break  can  be  discerned  between  ohalcocite 
of  the  graphic  areas  and  adjacent  ohalcocite  of  the  replace- 
ment type. 

Clearly  of  later  age  than  the  more  abundant  chaloooite 
Just  described  is  a  bluish  variety  of  the  mineral  in  tiny 
veinlets  cutting  the  bornite.  Where  these  cut  bornite-chal- 
cocite  graphic  areas,  it  is  sometimes  found  that  the  veinlet 
appears  in  the  bornite  lobes  but  is  not  distinguishable  in 
the  chalcocite  blebs.  With  the  chalcocite  in  these  veinlets, 
there  ia  a  small  amount  of  oovellite,  malachite,  and  limonite. 
A  little  kaolin  and  limonite  are  the  only  products  of  surface 
alteration  in  the  wall  rock.  The  slope  upon  which  the  ore- 
body  outcrops  is  very  steep,  and  almost  devoid  of  soil.  There 
is  no  zone  of  oxidized  material,  although  faint  limonite  and 


Si    »7.' 


~ 


'• 


139 


malachite  stains  extend  to  the  bottom  of  the  shaft.  The 
bornite  occurs  directly  at  the  surface,  with  only  the  merest 
traces  of  alteration. 

The  form  of  the  deposit  (viz.,  a  definite  lode  in  the 
diorite)  and  the  nature  of  the  associated  minerals  (including 
aotinolite,  epidote,  chlorite,  serioite,  and  heulandite)  in- 
dicate that  the  ore  is  epigenetic,  as  pointed  out  by  Turner, 
and  of  hydrothermal  origin.  The  early  formation  of  amphibole, 
the  dynamic  modification  of  the  rock,  the  subsequent  introduc- 
tion of  chlorite,  epidote  and  sericite  accompanied  by  bornite, 
and  the  development  of  heulandite  in  the  closing  phases  afford, 
together  with  the  absence  of  pyrite,  a  striking  similarity  to 
the  sequence  and  character  of  the  minerals  at  Engels.  The  oc- 
currence of  the  ore  in  a  well-defined  lode,  the  greater  abun- 
dance of  the  green  mica,  the  closer  relationship  between  the 
magnetite  and  bornite,  the  slightly  greater  rock  alteration, 
and  the  unimportance  of  products  of  oxidation  and  enrichment 
at  Superior  are  the  chief  points  of  difference. 

Conclusions  as  to  the  nature  of  most  of  the  chalcocite 
at  Superior  rest  on  the  interpretation  of  the  same  kind  of 
graphic  structures  between  bornite  and  chalcocite  as  are 
shown  at  Engels,  and  the  discussion  of  this  subject  on  pages 
]3>4-'imdl35  •Applies  to  both  deposits.  At  Superior,  however, 
it  is  ol v  .  that  the  chalcocite  in  these  forms  is  not  the 
product  of  enrichment  from  the  present  surface.  The  small 


. 


®     0  Jjt 

"'4 


,      .      •       c- 


amount  of  oxidation  under  the  active  mechanical  erosion  to 
which  the  deposit  is  now  subjected  may  be  measured  by  the 
feebleness  of  the  veinlets  of  blue  chaloocite  and  covellite 
and  the  alight  development  of  limonite  and  malachite.  Most 
of  the  chalcocite  is  clearly  of  an  earlier  age  than  the  vein- 
lets,  and,  if  a  product  of  descending  secondary  enrichment, 
it  must  have  been  produced  under  previous  topographic  condi- 
tions. The  surface  of  gentle  relief  in  this  immediate  neigh- 
borhood upon  which  were  deposited  the  extensive  Tertiary 
gravels  of  the  Jura  Eiver  appears  to  have  been  not  far  above 
the  present  surface  at  Engels,  while  perhaps  as  much  as  a 
thousand  feet  above  the  Superior  outcrop  of  to-day.  There  is 
distinct  evidence  of  at  least  two  later  stages  in  the  topo- 
graphic development,  respectively  shown  by  the  relatively  gen- 
tle slopes  and  open  valleys  of  the  higher  levels,  and  the  deep 
V-sliaped  canons  of  the  larger  streams. 

Down  at  the  Superior  deposit  where  the  deep  incision  of 
the  country  by  an  active  stream  permits  only  the  feeblest 
present  enrichment,  most  of  the  chalcocite  may  be  interpreted 
as  the  deep  roots  of  an  older  enrichment  formed  at  a  time  of 
less  violent  degredation.  At  Engels,  where  the  ore  has  not 
yet  been  reached  by  the  severe  erosion  of  the  present  cycle, 

1.  Diller,  J.S.,  Bull.  U.3.G.3.,  Wo.  353,  Plate  II. 


.leve 


LB  ^ 


there  is  a  moderately  thick  cover  of  oxidized  and  impover- 
ished material,  underlain  by  a  layer  of  rich  chalcocite,  and 
still  deeper,  by  plentiful  chaloocite  that  clearly  replaces 
bornite*  Although  the  vertical  range  of  the  Bngels  mine 
workings  is  too  small  to  afford  conclusive  evidence,  there 
is  a  distinct  indication  that  with  depth,  the  chalcocite  of 
undoubtedly  replacement  origin  decreases  in  proportion  to 
that  in  the  graphic  structures.  It  may  be  expected  with  a 
fair  degree  of  certainty  from  the  topographic  and  microscopic 
evidence  that  with  increasing  depth  at  Bngels,  the  character 
of  the  chalcocite  will  become  more  and  more  similar  to  that 
observed  in  the  Superior  ores.  In  other  words,  it  is  believed 
that  the  relations  of  the  chalcocite  and  bornite  exposed  in 
the  Superior  deposit  are  similar  to  the  relations  between  th« 
two  minerals  in  the  deeper  unexposed  portions  of  the  Bngels 
orebody.  It  is  to  be  noted,  however,  that  this  inference  is 
not  necessarily  dependent  upon  the  interpretation  given  the 
graphic  structure. 

SUMMARY. 

The  Bngels  deposit  presents  an  unusually  complete  record 
of  the  varied  conditions  to  which  the  rock  has  been  subjected 
from  its  initial  crystallization  to  the  last  feeble  hydrother- 
raal  changes  and  to  the  subsequent  alterations  by  surface  agen- 
cies. The  history  of  the  deposit  may  be  summarized  as  follows: 


JJtlfiT 

o 


142 


1.  Crystallization  of  a  basic  differentiate,  of 
noritio  character,  from  the  batholith  of  the  Sierra  Hevada. 
The  granodiorite  porphyry  also  probably  appeared  at  this 
time* 

2.  Iiocal  development  of  pegmatites;  also  pneumato- 
lytic  alteration  of  the  rook,  generally  widespread,  but  oc- 
casionally emphasized  along  seams,  producing  amphibole,  al- 
bite,  tourmaline,  magnetite,  and  some  sulphides,  accompanied 
and  perhaps  followed  by  straining,  and  partial  recrystalli- 
zation. 

3.  and  4.  More  localized  mineralization  and  altera- 
tion produced  under  hydrothermal  conditions,  intense  at  first, 

but  gradually  diminishing,  characterized  by  chlorite,  sericite, 

/ 

epidote,  and  sulphides  in  the  earlier  stages,  and  by  zeolites 
in  the  closing  phases,  with  increasing  dependence  upon  frac- 
tures. 

5.  Subsequent  cracking  of  the  rocks  and  ores* 

6.  Exposure  of  the  deposit  by  erosion  to  oxidation, 
with  the  production  of  an  impoverished  oxidized  zone  and  the 
development  of  secondary  sulphides. 

The  first  four  stages,  covering  the  time  from  initial 
magmatic  conditions  to  the  close  of  primary  mineralization, 
are  merely  convenient  divisions  of  one  uninterrupted  se- 
quence, while  the  fifth  and  sixth  were  separated  from  the 
others,  and  probably  from  each  other  by  indefinite  time  in- 
tervals. 


•xe&stf  J>eo 


In  the  primary  sequence,  the  presence  of  rainoralizers 
such  as  boron  and  either  chlorine  or  fluorine  is  indicated  at 
an  early  stage  by  the  occurrence  of  tourmaline  and  apatite, 
but  the  chief  mineralizer  from  the  beginning  to  the  end  was 
undoubtedly  water.  Its  influence  in  the  early  stages  is  sug- 
gested by  the  presence  of  amphibole,  and  the  development  of 
the  zeolites  is  conclusive  proof  of  its  presence  at  the  close. 
Much  of  the  rock  alteration  is  probably  merely  the  readjust- 
ment of  the  materials  of  the  rock  into  forms  more  stable  un- 
der the  influence  of  the  water  vapor  or  water  at  high  tempera- 
tures, without  notable  additions  of  new  elements,  except  the 
iron,  copper,  titanium  and  sulphur  to  form  the  ore  minerals. 
The  orebody  is  in  our  opinion  a  direct  result  of  igneous  ac- 
tion, but  was  formed  as  a  final  concentration  following  the 
crystallization  of  the  rock,  and  not  as  a  direct  magmatic 
segregation. 

Secondary  enrichment,  which  followed  the  exposure  of 

r 

the  deposit  to  oxidizing  influences,  accounts  for  all  the 
ohaloocite  present,  with  the  possible  exception  of  the  rela- 
tively small  amount  in  the  graphic  structure,  which  also  we 
are  inclined  to  attribute  to  downward  enrichment. 

These  conclusions  find  support  in  the  conditions  ex- 
hibited at  the  nearby  Superior  deposit  and  in  certain  physio- 
graphic features  of  the  region. 


• 


144, 


CONTACT  METAMORPHIC  DEPOSITS 
Seven  Devils,  Idaho. 

The  ores  of  the  Seven  Devils  district,  Idaho,  of  far 
definite  evidence  of  the  position  of  bornite  with  respeot  to 
the  silicates  formed  during  contact  metaraorphism,  and  present 
good  examples  of  several  important  structures  Of  bornite  and 
the  secondary  sulphides.  The  district  was  not  visited  in 
connection  with  this  investigation,  and  the  field  relations 

given  in  the  following  paragraphs  were  summarised  frora  descrip- 

1  2 

tionsby  Waldemar  Lindgren  and  by  J.  B.  Umpleby. 

The  rocfcs  of  the  Seven  Devils  district  consist  of 
elate,  quartz  ite,  limestone  and  large  amounts  of  aB«oniated 
greenstone,  intruded  and  altered  by  a  quarts  dlorlte  phase 
of  the  Idaho  granite.   The  series  is  capped  locally  by 
Columbia  basalt.   The  ore-deposits  occur  along  the  contact  of 
the  quartz  -iiorite  and  limestone  in  plaoe,  or  about  the 
narglns  of  included  blocks  of  limestone. 

The  most  abundant  gangue-minerals  are  garnet,  epidote 
and  quartz.  Bornite  and  chalcopyrite  are  the  chief  ore- 
minerals. 

In  three  mines  the  sulphides  occur  on  the  limestone 
side  of  the  garnet  zone.   They  are  connected  with  the  in- 
trusive by  means  of  veins  dipping  at  a  low  angle,  which  cut 


1.  w.  Lindgren.  c:0th  Anr..  Rep.,  U.S.a.S.,  Pt.  3,  pp.  ^4-y- 


2.  J.  B.  Umpleby,  The  occurrence  of  ore  on  the  limestone 
side  of  garnet  zones,  Univ.  of  California  Publication,  Depart. 
of  Geology  v.  Vol.  10,  pp.  2^-37,  (1915). 


aorotss  the  altered  Intervening  rock.  The  veins  have  been 
followed  by 


Quartz 
Jiorite 


limestone 


JO  feet 


Fig.  y.  Transverse  section  shoving  relation  of  ore  to 
r-net  zone,  Seven  Devils,  Idaho.1 


operations  only  to  the  diorite  contact,  but  they  are 
known  to  penetrate  the  igneous  roci.  for  at  least  25  or  £0 
feet  without  diminution. 

At  the  Peacock  iiine,  the  ore  occurs  in  the  central 
part  of  a  large  garnet-epidote  area  bordered  on  two  aides  by 
diorite  and.  on  the  south  by  an  engulfed  block  of  greenstone. 
A  drill-core  chows  the  centre  of  the  mineralized  area  to  con- 
tain unaltered  limestone  at  depth,  thus  affording  another 
example  of  the  occurrence  of  sulphides  between  the  limestone 
ir.l  the  contact  silicate  rooks. 

rrom  the  microscopical  study,  the  sequence  of  ganque 
and  ore-minerals  may  be  established  in  a  general  way  as 
follows.  Orthoclase,  amphibole,  pyroxene  (diopside),  garnet 
aiio  :i|.itite  ooristituts  a  ^roup  fornod.  under  the  early  in- 
tense phases  of  the  mineral  iz  at  ion.  Zpidote  and  titanite, 


1.  J.  B.  Umpleby,  Ibid 


146. 


are  probably  somewhat  later  In  origin,  and  chlorite  lags  still 

further  behind.   Quartz  and  oalcite  were  formed  from  the  be- 

/•. 

ginning  to  the  end  in  various  amounts.  Oalcite,  however, 
is  most  prominent  as  a  late  product,  and  is  common  as  a  re- 
placement of  epidote,  the  feldspars  and  other  earlier  min- 
erals. 

The  sulphides,  bornite  and  chaloopyrite,  are  later 
than  the  first  group  of  minerals,  and  later  than  the  ep- 
idote,  but  they  are  probably  in  part  contemporaneous  and  in 
part  earlier  than  the  chlorite,  quartz  and  oalcite.  These 
relations,  :mp^ortsd  by  the  field  distribution  of  the  ore- 
minerals,  indicate  that  the  sulphides  were  formed  during 
milder  phases  of  the  mineralization,  than  those  under  which 
the  high- temperature  contact  silicates  were  deposited. 

The  amount  of  secondary  alteration  which  the  ore  has 
suffered  is  not  great,  but  there  IB  a  small  amount  of  chal- 
ooc ite,  qovellite  and  ohalcopyrlte  of  secondary  nature  in 
nearly  all  the  specimens  of  bornite  studied.  The  chalooclte 
occurs  for  the  most  part  in  snail  irregular  velnlets  or  rlns 
about  bornite  grains  and  in  thes<3  forms  can  be  attributed 
without  henitation  to  the  agency  of  descending  surface  solutions 
It  also  occurs,  however,  to  a  minor  extent  in  graphic  struct- 
ures with  the  bornite  and  in  a  few  larger  patches  in  massive 
bornite  in  which  it  ia  less  certainly  of  replacement  origin. 
In  these  forme,  the  chalooclte  apparently  possesses  the  ortho- 


147 


horabie  structure,  for  in  a  few  oases  the  character  let  ic 
pattern  of  straight  lines  oriented  parallel  to  one  direction 
ie  shown  by  the  etch-cleavage  and  by  malachite  veinlets. 

The  structure  of  the  chalcocite  in  the  riiaa  and 
velnletn  is  less  definitely  shorn  by  etching,  but  lattice 
patterns  are  frequently  formed  in  it  by  the  development 
of  covellite  and  oxidized  products.   The  replacement  of 
bornite  by  chaloocite  yields  Imperfect  lattice  structures  in 
a  few  planes,  but  it  in  not  a  common  relation  between  the  two 
minerals  in  thesr?  ores. 

Chalcopyrlte  ie  abundant  as  fine  plates  in  the 
bornite,  vrhich  form  usually  well-developed  lattice  patterns 
on  the  polished  surface.   The  chalocpyrite  is  more  widely 
distributed  than  the  chalcocite,  but  it  is  clearly  related  to 
the  same  ohannel-waya  and  is  without  doubt  the  product  of 
the  same  solutions.  The  relations  are  very  similar  to  thoae 
previously  described  in  the  ores  from  Engels,  La  Fleur  Mountain, 
and  Copper  Mountain  in  the  Slrnilfcaiaeen  region.   In  some 
grains,  the  plates  of  ohalcopyrlte  are  BO  fine  and  eo  cloaely 
spaced  that  they  can  be  detected  only  with  difficulty  under 
the  highest  magnifications,  and  it  IB  very  probable  that  the 
peculiar  yellow  tint  of  certain  spots  in  the  bornite  from 
Seven  Devils  is  due  to  sub-microeoople  chalcopyrite  of  this 
character. 

Plates  of  covellite  aeouna  lattice  orientation  in 
both  the  bornite  and  in  the  derived  chaloocite.   In  places, 


148, 


the  oovellite  In  an  intermediate  product  in  the  replace- 
ment of  ohalcocite  by  malachite. 

In  summary,  the  deposits  at  Seven  Devils  are  believed 
to  offer  very  convincing  evidence  both  from  the  field  and 
.  i'.roscopic  relations  that  the  bornite  is  later  than  the  high- 
temperature  contact  silicates.  The  development  of  chaloo- 
pyrite,  in  exceedingly  minute  lattices,  and  its  dependence  on 
the  development  of  secondary  chalcooite  and  oovellite  is 
exceptionally  well  shown,  and  the  inheritance  of  structure  of 
ths  bornite  by  the  chalcocite  is  strongly  suggested  by  the 
formation  of  covellite  in  the  lattice  structure  in  the  chal- 
cocite. 

The  origin  of  the  small  amount  of  chalcocite  in  the 
graphic  structure  can  not  be  settled  from  the  evidence  in 
this  case  alone,  and  will  be  licousssd  in  a  nore  general 
way  in  a  later  part  of  the  pr.per. 


149. 


The  ffhitehorr.e  District.  Yukon  Territory.. 

In  the  fiontact-zaetamorphlc  ores  of  the  Whitehors« 
District,  YuKon  Territory,  bornite  is  the  most  abundant 
sulphide.   The  district  was  not  studied  In  the  field  in 
connection  with  thip  \vorfc,  but  from  information  gained  from 
R.  Q.  McCormell^s  excellent  descriptions,  and  from  an  exam- 
arainntlon  of  material  in  the  collection  of  the  Massachusetts 
Institute  of  Technology  the  occurrence  is  believed  to  be  of 
sufficient  value  <js  sn  example  of  bornite  formed  under  contact- 
netamorphlc  conditions  to  be  worth  summarizing. 

The  district  in  situated  In  the  valley  of  the  Lewes 
River  in  the  southern  part  of  Yu&on  Territory  near  the  town 
of  Whitehorae,  the  teriainua  of  the  White  Pass  and  Yuion 
Rail  Road,  which  affords  conraiur.io.it ion  with  Skagvay  on  the 
Lynn  Canal,  110  miles  to  the  south. 

The  various  formations  in  the  copper  belt  may  be 
tabulated  as  follows: 

Pleistocene  -  Silts,  boulder  clays. 

Tertiary  -  Basalts. 

Mesozoio  -  Porphyry  difces,  granites,  porphyritee. 

Carboniferous  -  Limestone. 

Of  those  rocks,  only  the  limestone  and  the  plutonio 
intrueivee  are  of  interest  in  connection  v*ith  the  ores. 

1.   The  Whitehorae  Copper  Belt,  YuXon  Territory,   Report 
No.  1050,   O.G.S.,  (  1909). 


150 


The  earlier  porphyrites  are  rarely  mineralized,  and  the  por- 
phyry dikes  are  always  barren  and  clearly  post-ore. 

The  granitic  rocks  are  believed  to  be  outliers 
of  the  Coast  Range  batholith.  They  are  extraordinarily 
varied  ir.  their  raineraloglcal  composition,  ranging  from 
hornblende  granite  to  diorites,  hornblende  and  auglte  syenites 
and  even  to  gabbros.  The  commonest  phase  is  the  hornblende 
granite.   It  is  a  medium  to  coarse-grained  rook  consisting 
essentially  of  quartz  in  grains  of  various  size,  orthoclase 
and  hornblende.  A  little  andeslne  or  ollgoclase,  auglte  and 
biotite  are  usually  present.  Magnetite,  ilmenite,  titanlte 
and  apatite  are  the  less  abundant  accessories.  Mioro- 
pegmatitic  intergrowths  of  quartz  and  feldspar  are  mentioned, 
but  are  stated  to  be  of  uncommon  occurrence. 

The  limestones  are  moderately  crystalline,  dark 
greyish  rooks.  Argillaceous  or  dolomitio  bands  are  absent. 
Small  oherty  aggregates  are  the  chief  impurities  mentioned. 
The  limestone  remains  only  as  irregular  residues  in  the  midat 
of  the  wide-spread  Intrusive  rocks  which  have  penetrated  the 
region. 

The  mineralization  is  confined  to  a  belt  ext-snding 
12  miles  or  so  in  a  north-northwest  direction,  but  seldom 
over  one  mile  in  width.  The  ore-bodies,  which  are  exceedingly 
irregular  in  distribution  follow  a  series  of  limestone  in- 
clusions in  the  igneous  rocks.   The  uietamorphic  all  lea  tea  and 


151 


the  sulphides  are  developed  both  in  the  limestone  and  in  the 
granite.  The  contact  silicates  are  sven  more  abundant  in 
the  granite  thin  in  the  limestone,  and  in  many  places  the 
contact  between  the  two  rocks  is  completely  obliterated. 
From  the  descriptions  and  from  the  relations  shown  on 
UcConnell»s  map,  the  rich  sulphide  ores  apparently  end  abrupt- 
ly against  recrystallized  line at one.  an  one  side,  and  pass 
gradually  into  contact-altered  limestone  or  granite  on  the 
other.   The  distribution  of  the  ores  offersmany  striking 
examples  of  the  occurrence  of  sulphides  between  limestone  and 
rocks  rich  in  contact  minerals. 

The  principal  ore-minerals  are  magnetite,  hematite, 
bornite  and  chalcopyrite.   Other  minerals  of  less  common 
occurrence  are  arsenopyrite,  tetrahedrite,  galena,  sphalerite, 
molybdenite,  antf  chalcoclte  .  Gold  and  silver  occur  in  some 
quantity  in  most  of  the  deposits.  Pyrite  is  found  in  scat- 
tered grains  in  the  granite  but  rarely  occurs  in  connection 
with  the  ores. 

The  important  contact-silicates  mentioned  by  MoConnall 
are  garnet  (andradite),  augite,  tremollte,  aotinolite,  ep- 
idote,  calcite,  clinochlore,  scapolite,  serpentine  and  quartz. 
To  these  may  be  added  from  the  study  of  other  material, 
wollastonite  ,  and  diopeide 

According  to  Stutzer,  the  succession  is  pyroxene- 
magnet  ite-garnet-e  ale  ite-sulphide*.  Tho  bornite  is  clearly 

1.   Zeltschr.  f.  praXt.  Oeol. ,  17,  1909,  pp.  116-120. 
Lindgren,  Sineral  Deposits,  New  York,  1913,  P. 


152, 


ln,ter  than  garnet  or  pyroxene,  and  probably  later  than  ep- 
idote  in  material  studied  microscopically  in  the  collections 
available.  Calcite,  hov.'ever,  is  deposited  in  part  at  least 
later  than  the  sulphides. 

In  the  ores  rich  in  iron,  the  bornite  and  ehaloo- 
pyrite  occur  both  in  scattered  blebs  in  the  magnetite, 
apparently  of  contemporaneous  age,  and  as  bands  and  patches 
between  the  magnetite  grains  in  the  r6le  of  a  cementing 
material.   The  most  intense  development  of  the  copper  sul- 
phides was  clearly  later  than  the  magnetite.  The  bornite  and 
chalcopyrite  are  associated  with  mutual  boundaries,  and  in 
a  broad  way  they  are  probably  contemporaneous. 

Malachite,  azurite,  cuprite,  tenorite  and  ohrys- 
ocolla  are  present  in  the  oxidized  ores.  Limonite  is  also 
common.  Ths  ores  rich  in  magnetite  are  little  altered  at  th« 
surface.   The  sulphides  in  the  rocks  rich  in  contact  sil- 
icates are  usually  resistant  to  oxidation,  but  in  the  lime- 
stone the  alteration  to  oxidized  minerals  may  be  fairly  com- 
plete to  a  depth  of  100  feet.  A  small  amount  of  ohalcocite 
is  developed  as  a  replacement  of  bornite,  and  to  a  less  ex- 
tent, as  a  replacement  of  ohalcopyrite.  Covellite  occurs 
but  it  IB  uncommon. 

Summary.  The  bornite  ores  of  the  Whitehorse  dis- 
trict are  of  distinct  cor.tact-metamorphic  origin.  The  bor- 
nite and  the  associated  ohalcopyrite  were  formed  later  than 


153, 


the  intense  phases  of  the  minerallz  at  ion  which  produced  the 
high- temperature  silicates,  (garnet  and  pyroxene).  The 
sulphide  ore-bodies  are  situated  for  the  most  part  between 
intensely  altered  limestone  and  naraorized  but  otherwise 
unaltered  limestone.   There  is  noteworthy  oxidation  and 
a  alight  development  of  chalcocite  apparently  related  to  the 
present  surface,  which  Is  significant,  for  the  severe  cli- 
matic conditions  which  prevail  in  this  region  at  present, 
and  have  prevailed  in  the  recent  geological  past,  are  not 
favorable  for  superficial  changes. 


154 


TH1;  MARBI£  BAY  MIflE .  TEXADA  loLAHD.  B.  C. 

INTRODUCTION. 

The  ore  at  the  Marble  Bay  Mine,  Texada  Island,  consists 
of  irregular  bodies  of  sulphides,  apparently  formed  by  the 
replacement  of  limestone  in  the  neighborhood  of  small  in- 
trusions of  dioritio  rooks •  The  oontaot-metamorphio  character 
of  the  deposit  is  clearly  shown  by  the  geologic'  relations, 
and  especially  by  the  nature  of  the  accompanying  rock  al- 
teration. Chaloopyrite  and  bornite  are  the  chief  ore- 
minerals,  .vith  garnet,  diopeide,  epldote,  veuuvianite  and 
wollastonite  abundantly  developed  in  the  adjacent  limestone* 

Situation.    Teacada  Island  lies  in  the  Gulf  of  Georgia 
about  fifty  miles  northwest  of  the  city  of  Vancouver.  The 
Island  is  about  30  miles  in  length,  end  averages  roughly  3 
miles  in  width.  The  Marble  Bay  Mine  is  situated  at  an 
elevation  of  55  ft.  near  the  port  of  Vananda  at  the  northern 
end  of  tho  island* 

Development.   There  are  several  deposits  in  the  neigh- 
borhood of  Vananda  which  have  been  worked  in  an  intermittant 
way  for  a  number  of  years,  !>ut  vTlth  the  exception  of  the 
Marble  Bay  ::ine,  the  irregularity  in  form  and  distribution 
of  the  ore -bodies  has  made  the  results  distinctly  discouraging. 
In  the  liarble  Bay  i^ine,  however,  niuoh  larger  masses  of 
sulphides  have  been  encountered  with  depth  than  ,-ould  have  been 
expected  from  the  scanty  surfftQe  ohowings,  and  until  the  last 
few  years  the  annual  shipments  have  ranged  from  12000  to  15000 
tons  of  ore,  varying  from  3  to  11  percent  copper  with  1-5  oz* 


iae'C 


155, 


1 
of  silver  and  .1  to  .7  oz.  of  gold  per  ton.   The  mine  is 

opened  by  meane  of  a  shaft  about  1300  ft.  deep.  The  ore 

occurs  in  a  zone  of  discontinuous  irregular  bodies  pitching 

to  the  northwest,  which  carries  them  from  the  neighborhood 

of  the  shaft  on  the  upper  levels  to  several  hundred  feet  to 

the  north  and  west  in  the  lower  v/orkings.  The  deepest  level 

(July,  1916)  Is  termed  the  15th,  and  is  about  1360  ft.  below 

the  collar  of  the  shaft,  and  a  little  over  1300  ft .below  sea-level 

Literature.  The  most  complete  description  of  the  geology 
and  ore -deposit s  of  Texada  Island  is  a  report  by  B.  6.  MoConnell, 

published  as  a  memoir  of  the  Canadian  Geological  Survey.  In  it, 

2  3 

previous  work  of  the  Survey  by  James  Biohardson,  G.  M.  Dawson, 

4 
end  0.  E.  LeBoy  is  summarized.  The  bornite  ores  of  the  island 

have  also  been  described  by  ff.  M.  Brewer  in  a  general  article 

on  the  subject  of  bornlte-chaloopyrlte  deposits  of  the  ooast- 

5 
region.   Le  Boy's  views  of  the  ore-deposit  at  the  Marble  Bay 

G 
:.!ine  are  published  in  a  later  article  as  well  as  in  the  earlier 

4 
survey  report  (1906)  and  they  are  essentially  the  same  as  those 

7 
expressed  in  greater  detail  in  McDonnell's  memoir. 


1.  R.  C.  MoConnell,  Teradft,Island  B.C.,  Mornoir  58,C.G.S.,1914. 

H.  Jaraes  Hiohardson,  Ann.  Ttoport  C.G. 3.,  1873-74,  pp  99-100. 

3.  G.M.  Dawson,  (1885)  Annual  Beport  C.G.S.,Vol.  Il.pp. 36-37-B- 

4.  }«]  .  Le3oy,  Preliminary  Report  on  a  Portion  of  the  ^l 
Coast  of  B.C.  and  Adjacent  Islands, C.G. 3.,  1909. 

5.  .7.  :i.  Brewer,  Jour.  Can.  Mine,  Inst., 

6.  R.  G.  MoConnell,  Loo.  oit. 


156, 


1 
GKHERAL      )QT 

A  series  of  voloanios  and  interbedded  limestones  known 
as  the  Anderson  Buy  formation  are  the  oldest  rooks  recognized 
on  tho  island.  They  are  overlain  "by  crystalline  limestones 
of  Trlassio  age  termed  the  Marble  Bay  formation,  which  is  of 
distinct  economlo  importance  as  it  is  the  chief  ore-bearer  of 
the  copper  region.  Both  formations  are  broken  by  extensive 
intrusions  of  variable  nature  called  porphyrites,  v;hloh  are 
the  most  widely  distribute,  rooks  of  tho  inland.  These  are 
in  turn  Intruded  by  quartz  diorite,  diorite  and  diorlte 
porphyry  which  are  believed  to  be  local  manifestations  of  the 
great  igneous  invasion  at  the  close  of  the  Jurassic  that  formed 
the  Ooast  Ranpe  bathollth.  The  quartz  diorite  and  diorite  form 
email  stocks, but  are  not  known  In  larger  bodies  on  tho  island* 

V 

A  number  of  small  intrusions  of  the  diorite  occur  near  the 
karble  Bay  -line,  and  are  clearly  the  source  of  the  mineralization* 
Diorite  porphry  dikes,  either  ae  direct  apophyses  of  tho  stocks, 
or  more  widely  distributed,  are  very  common.  They  are  fre- 
quently encountered  in  the  workings  of  the  mine  and  apparently 
increase  in  abundance  with  depth*  The  mineralization  is  be- 
lieved by  MoConnell  to  have  followed  the  period  of  dike  intrusion* 

Ho  record,  of  igneous  activity  leter  than  the  Jurassic  Is 
found.  In  the  Cretaceous,  a  aeries  of  conglomerates,  sandstones 

1.  Summarized  from  II.  G.  MoConnell 's  Memoir. 


157 


and  shales  were  formed,  but  only  remnants  remain  In  protected 
depressions.  The  Tertiary  is  entirely  a  period  of  degradation- 
At  the  time  of  maximum  glaclatlon  in  the  Pleistocene  the 
island  was  completely  covered  "by  southeastwardly  moving  io«. 
During  pert  of  this  period  tho  island  was  submerged,  and 
interplaoial  clays,  sands  and  silts  were  deposited  along  its 
coast.  Glacial  deposits  containing  narine  fossils  have  been 
found  on  the  island  at  an  altitude  of  500  ft.,  and  are  be- 
lieved to  indicate  a  post -glacial  uplift  of  that  magnitude. 
Since  the  retreat  of  the  ice,  and  the  rise  of  the  land,  con- 
ditions have  been  stationary  for  t  sufficient  time  to  permit 
occasional  wide  rock  beaches  and  low  cliffs  to  have  b  >en  out 
in  the  resistant  quartz  diorlte  or  limestone.  The  southern 
part  of  tho  island  composed  ohiafly  of  porphy rites, has  been 
left  ^  region  of  notable  rolief ,  .vith  peaks  rising  to  a 
maximum  elevation  of  £892  ft.,  while  the  northern  part,  com- 
posed of  the  softer  sediments,  has  been  worn  by  the  long  con- 
tinued erosion  to  a  milder  country  of  basins  and  rough  sea- 
ward sloping  plains. 

ROCK3  AMD  ORSS  AT  THE  MABBLE  BAY  MINE 
Limestone.   The  Marblo  Bay  limestone  IB  an  obscurely 
bedded  rook,  grey  or  white  in  color,  and  in  plaoes  of  sufficient 
purity  to  be  of  commercial  value.  In  the  viainity  of  the  ore- 
deposits,  however,  it  becomes  marmorized,  and  impure  with  various 
contact-metarnorphic  silicates.  Near  the  mine,  no  trac^  of  bedding 
can  be  detected  in  it,  although  in  plaoes  there  is  a  curious  fine 
banding. 


158. 


Porphyry*  Three  small  stocks  of  rather  basic  diorite 
and  dlorlte  porphyry  outorop  in  the  neighborhood  of  the 
mine*  numerous  apophyses  from  these  "bodies  are  associated 
with  the  ore,  and  below  the  13th  level,  masses  of  stock- 
like  proportions  are  encountered  by  the  mine  workings* 
::icro8oopical  examination  of  cpeoiaiens  from  the  15tn  level, 

i 

show  the  rook  to  be  composed  of  andeeine  and  hornblende 
phanoorysts  in  a  microrranltio  ground-mass  of  andeeine  " 
,vith  some  oligoolase  and  orthoclase*  The  feldspar 
rhenooryets  occur  in  sharpl^  ^uheflral  forms,  and  in  many 
oases  are  strikingly  zoned,  ranging  from  cores  as  basic  as 
medium  labradorite  to  rime  as  acid  ae  oligoolase.  Tho  horn- 
blonde  io  oommonly  in  small  prisms*   Some  dikes  are  distinctly  dark* 
due  to  a  reator  abundance  of  hornblende*  A  small  amount  of 

biotite  occasionally  accompanies  the  hornblende.  Small 
crystals  of  apatite,  a  little  titanite  and  some  quartz  are 
usually  present  as  accessories.  The  Igneous  rooks  are  fairly 
fresh,  "but  show  a  slight  development  of  the  contact -metamorphio 
minerals* 

SerioitA  in  the  feldspars,  and  oalolte  and  chlorite  as 
replacements  of  the  horiblende  ho\v«v«"  «-«  common,  even  when 
the  rook  is  apparently  freeh.  In  a  few  instances,  the  alteration 
of  the  dikes  ie  intense  and  accompanied  by  the  introduction 
of  sulphides*  In  these  cases  ch&lcopyrite  usually  predominates; 
bornite  is  rare. 


159, 


Alteration  of  the  limestone.  The  development  of  garnet, 
diopside  and  vesuvianlte  in  the  limestone  was  the  earliest 
phase  of  the  mineralization.  The  exact  age  relationships 
of  the  minerals  are  difficult  to  decipher,  but  it  is  fairly 
probable  that  the  vesuviar.lte  was  earliest,  and  the  garnet 
and  the  diopside  i'ougrhly  contemporaneous.  The  vesuvlanlte 
is  less  abundant  than  the  others,  but  it  Is  prominent  in 
certain  parts  of  the  ore,  and  occurs  often  in  largo  well- 
formed  prisms,  set  in  a  cement  of  the  later  sulphides.  Tha 
garnet  is  the  yellow-bro      andradite ,  usually  in  granular 
masses,  but  In  places  well-crystallized  in  small  rhombic 
dodecahedrons.  The  diopside  is  much  finer  in  grain,  and  la 

t 

usually  observed  megascoploally  only  as  dull  green  compact 
material,  making  up  a  notablo  part  of  the  altered  limestone. 
Under  the  microscope,  it  is  commonly  seen  to  be  In  email  but 
in  many  cases,  euiiedral,  crystals,  readily  identified  by  their 
pyroxene  cleavage,  or  in  very  fine  shapeless  grains  comprising 
the  chief  mass  of  the  rook.  It  is  probably  of  eoual  quantita- 
tive importance  <7lth  the  garnet,  "but  as  It  Is  lass  easily 
recognized,  the  garnet  appears  far  more  abundant  in  the 
field.  Large  masses  of  coarse  laths  of  wollastonlte  are 
occasionally  found,  and  fine  laths  of  the  mineral  are  fairly 
common  under  the  microscope.  Tremolite  occurs  -vith  It  but 
It  is  much  less  abundant.  The  two  minerals  are  closely 
associated  with  diopside  end  garnet,  und  are  probably  of  an 
eerly  age. 


160, 


Epldote,  chlorite,  sorlcito,  ;;nd  quartz  form  a  seoond 
group  clearly  Inter  than  the  minerals  just  described,  since 
thoy  are  found  chiefly  as  replacements  of  tho  earlier  products. 
With  these  later  minerals,  tha  sulphides.!  seem  most  closely 
associated,  although  in  part  the  oros  follov;  them, for 
euhedral  or  corroded  inclusions  of  even  these  gangue  minerals 
have  been  observed  in  the  bornite  and  chalcopyrito.  The 
epidote  for  the  most  part  IE  the  colorless  variety  low  in 
iron.  It  is  optically  both  positive  and  negative,  but  still 
beyond  the  range  01'  clinozoieite.  It  fonas  broad  bands  in 
the  altered  limestone,  and  in  places  clearly  replaces  tht 
earlier  garnet,  and  vesuvianite.  Sericite  and  chlorite  are 
not  abundant,  but  occur  as  replacements  of  feldspars  and 
hornblende  In  the  porphyries,  or  as  seams  in  the  other 
metamorphio  minerals*  Both  are  abundant  in  certain  bornite 
and  ohaloopyrite  grains,  usually  near  the  margins.  Quartz  is 
uncommon  but  occurs  in  small  amounts  in  various  relationships. 
Calcite  is  abundant,  both  as  rocrystallized  grains  of  the 
limestone  and  as  vein lots  and  masses  of  replacement  origin, 
developed  at  tho  expense  of  the  contact  minerals. 

One  of  tho  most  unusual  features  is  the  occurrence  of  the 
zeolites,  heulandite  and  laumonite,  in  grains  vvith  caloite  and 
metaaorphio  minerals,  or  as  fine  seame  cutting  all  other  con- 
stituents. Their  total  amount  is  ouite  small,  but  they  seem 
fairly  v.-idely  distributed.  The  lauaionite  was  present  in 
sufficient  quantity  in  one  specimen  from  a  garnet  zono  on  the 
15th  level  to  permit  the  identification  in  the  thin  section 


«> 


to  be  checked  by  means  of  index  liquids.  Certain  other 
minerals  of  low  relief  were  observed  which,  may  be  other 
zeolites,  but  a  satisfactory  determination  was  not  possible 
on  account  of  the  small  amount  of  material, 

Ore  Minerals*   Chalcopyrite  and  bornite  &ro  the  chief 
sulphides.  Prom  the  impression  gained  underground,  uhalcopy- 
rite  is  more  abundant  at  present,  but  from  descriptionaof 
the  larger  ore-bodies,  now  mined  out,  it  ie  probable  that 
bornite  was  the  most  important  ore-mineral.  There  is  a  small 
amount  of  pyrite,  but  usually  not  in  the  bornite  ores.  The 
pyrite,  in  the  few  polished  chips  where  it  was  observed,  is 
the  earliest  sulphide.  Galena,  steinmannite ,  sphalerite, 
tetrahecirite ,  and  klaprotholite  all  occur  in  subordinate 

amounts*  A  little  pyrrhotito  was  reported  near  a  dike  on  the 

1 
7th  level,  but  it  hao  not  been  recognized  elsewhere.  Hone 

was  observed  under  the  microscope.   Molybdenite  occurs  in 
small  amounts  from  the  cropping  to  the  lowest  levels.  Its 
relation  to  the  copper  minerals  was  not  detected.  A  few  grains 
of  a  hard,  somewhat  pinkish  mineral  were  observed  under  the 
microscope,  but  it  could  not  be  identified  by  microchemioal 
means.  Native  silver  is  said  to  be  common  in  various  parts  of 
the  mine.   It  occurred  most  abundantly  in  the  large  ore-body 
above  the  13th  level.  Native  gold  has  been  reported  but  it  is 
very  rare. 

The  cheloopyrlte  and  bornite  are  intimately  associated; 
their  boundaries  are  commonly  of  the  mutual  type,  but  as  usual 

1.  R.C.iloConnell.  Loc.  oit.  ID. 51. 


there  is  a  littlo  evidence  that  the  bornite  was  slightly  later, 
or  at  least  continued  to  fora  longer  and  corroded  tho  ohalcopy- 
rite.  Thero  are  few  definite  bornite  veinlets  in  tho  ohaloopy- 
rite,  nut  inoro  numerous  torifuoe  end  embayments  of  bornite  tend 
to  surround  and  isolate  chaloopyrite  grains  than  the  reverse, 
and  in  general  the  secuence  is  fairly  clear.  (Pig.  50  )  In  a 
far;  instances,  tho  relations  between  the  two  minerals  suggest 
the  graphic:  structure*  Sphalerite  in  rather  large  grains  is    .'. 
not  uncommon.  It  seems  contemporaneous  v.ith  the  ohaloopyrite 
ae  some  fine  grained  intergrowths  of  tho  two  minerals  wore 
observed  and  it  is  slightly  aorroded  by  the  bornite* 

In  addition  to  the  forms  Just  described  email  specks  and 
spines  of  chalcopyrite  occur  in  the  bornite,  in  some  oases 
In  fine  lines  of  small  blebs  apparently  along  the  contacts  of 
grains.  Selena  and  tetr&hedrito  are  also  common  In  this 
relationship,  "hut  in  addition  both  of  these  minerals  form 
minute  end  intricate  ^rephlc  structures  v/Jth  the  bornite,  which 
are  most  readily  interpreted  ae  contemporaneous  intergrowths, 

t 

Aj-eocie-ted  vith  the  galena,  and  not  readily  distinguishable  from 
it  except  chemically  is  the  arsenical  antitnonial  variety, 
stoinmannite.  It  is  abundant  in  galana  fror,  the  l?th  level. 

The  small  amount  of  a  mineral,  somewhat  doubtfully  identified 

1 
as  klaprotholite- .VBS  observed  in  the  bornite. 

1.  Klaprotholite  '.vat-  first  determined  in  OOT>J  er  ores  by  P»B.  Laney 
in  tnatorial  fro.a  Butte.  It  has  al-^o  been  obsorvec  b,y  A.F.r.orrers  in 
ores  i'rora  several  different  localities. (see  Koon.Oeol.  Vol.;T,p.5i3  5 
,>n  the  policho-  surface  the  nlncral  in  the  Taxada  ores  has  a  bright 
creainy, silver-  .-hite  color,  and  is  distinctly  softer  than  the  sur- 
rounding bornite.  Its  microchemical  properties  are  as  follov/s:- 
with  dilute  HiJO^i  brownish  black, poreistant :  ;;ith  dilute  or  co-nc, 
H  Cl,  brown  thoa  black,  rubs  to  a  gray  surface:  ,vith  ?.C3,  no  re- 


163 


The  mineral  ir.  in  round  or  elongated  blebs  for  the 
most  part,  "but  in  a  few  instances  its  vein-like  form 
makee  it  very  probable  that  it  is  a  replacement  of  the 
bornite.   In  some  cases,  it  forms  eriali  irregular  foathery 
masses  alonp  the  margins  of  bornite  areas.  The  specimens 
in  vhioh  it  was  observed  were  from  a  large  ore-body  on  the 
10th  level.  (  ?lg.  '4-9). 

Secondary  Sulphides.    Chaloocite  and  oovelllte 
occur  in  very  small  amounts  as  deep  as  the  10th  level. 
They  are  usually  present  only  as  vo inlets  of  thread-like 
dimensions,  "hut  in  one  specimen  noteworthy  rima  of  ohalco- 
cite  folio-,  the  ed^es  of  bornito  grains.  Jhe  bornite  is 
the  only  sulphide  in  v.-hioh  the  veinlets  are  at  all 
prominent.  Fuzzy  rosettes  of  oovellite  penetrate  the 
bornite  from  certain  ragged  patches  of  included  >?angue 
minerals,  and  are  identical  in  form,  and  appearance  with 
oovollite  known  to  have  been  +'ound  by  descending  proceeees. 
In  the  chalcorite  rims,  in  the  case  mentioned  above,  there 
is  a  notable  amount  of  the  klaprotholite,  in  ragged  laoy 
forms, partially  altered  to  a  dull,  blue-rray  product,  which 
in  turn  is  surrounded  by  the  ohalcoclte.  There  it-  a 
distinct  suggestion  that  the  klcr>rotholite  former  un  inter- 
mittant  rim  about  the  bornito  arer.e,  and  that  it  became 
include*^  in  the  ch&lcooite  v/hich  replaced  the  Bornite.  The 
chalooolte  ae  -"or  as  observed  does  not  develop  the  lattice 
structure  with  the  bornite. 


.( '•    - 


-IJ-B 


164. 


The  ores  on  the?  uppor  levels  have  been  wor    .ut,  and 
the  stopee  ero  not  acoeuaible,  but  there  was  eaid  to  be  no 
marked  zone  of  secondary  oroB  or  of  complete  oxidation. 
Primary  sulphides  o.sour  within  a  few  feet  of  the  surface. 
On  tho  other  hand  a  small  amount  of  azurite  oceure  with  the 
chalcooito  in  bornito  in  a  specimen  eald  to  have  como  from 
the  10th  level*   It  replaces  the  bornite  in  otaall  peculiarly 
anpular  patches  and  :rrcine. 

Although  1500  ft.  below  sea-level,  the  lower  workings  of 
the  mine  are  quits  dry.  Surface  waters  enter  the  mine  along 
tn  open  fracture  termed  the  Llud-ellp,  but  eecept  during  ex- 
ceptionally wet  seasons,  the  flo«v  is  caught  on  the  upper  levels 
.v.nd  pumper  back* 

r-ucussioa 

Distribution  and  origin  of  the  ore  -bodies*  The  relations 
between  the  sones  of  altered  limestone  and  tho  ore-bodies  are 
complex,  and  difficult  to  state  in  a  genorei  way*  There  is, 
however,  a  distinct  suggestion  that  the  ores  occur  most 

between  garnotized  limestone  and  pure  recrystallized 


or  marinorized  limestone,  i'his  is  shown  by  Mc.Connell's  maps 

1 
of  the  levels*     In  many  jasos,  however,  tho  sulphides 

apparently  occur  in  unaltere,  marble,  but  in  most  of  these 

ti'-noea,  ;;arnetized  rook  would  probably  be  found  in  contact 
.vith  the  oro  on  tho  sides  "Solow  tho  section  shown  on  the  maps* 

Prom  the  promising  results  in  similar  occurrences  studied  by 

2 
A*  Locke  and  ii  .  ri.  Perry,  and  from  the  valuable  generalizations 


1.  Op*  cit* 

2,  Oral  cor.  .unioation. 


165 


1 

by  J.  B.  Uiapleby  on  the  distribution  of  sulphidas,  j 

limestone  and  iinaltereo  limestone  in  deposits  of  this  sort, 
it  is  very  prooable  that  a  thorough  study  of  the  distribution 
of  ores  v/ith  respect  to  altered  limestone  would  yield  results 
of  definite  commercial  valuo  in  directing  further  prospecting 
for  new  ore-bodies. 

There  seems  to  be  little  significance  in  the  relations 
of  the  ore-bodies  and  the  numerous  dikes.  Both  are  probably  ' 
manifestations  of  the  same  igneous  tiCtivity,  but  it  seems 
probable  that  the  ores  arc  later,  as  the  dikes  themselves 
become  mineralizes  and  altered.  They  may  have  had  a  directing 
action  on  the  ;aineruli;:ing  solutions  however,  which  v.ould  be 
•.vorthy  of  study.  McConncll  suggests  that  the  emanations 
which  produced  the  ores  were  derived  from  deeper  portions 
of  the  magma  tna  not  directly  from  the  rocks  exposed  at 
present.  The  rielng  solutions  were  probably  directed  to  a 
certain  extent  by  the  limestone  -  prophyry  contacts. 

::inerr.l  se  cue  nee.  The  seouenoe  of  the  minerals  of  the 
deposit,  based  on  evidence  obtained  by  the  microscope  for  the 
most  part,  ie  expresses  graphically  on  page  (167).  ihe  details 
of  the  grouping  are  of  course  only  a  rough  approximation,  but 
certair  broad  features  may  be  regarded  v/ith  certainty.  The 
sulphides  are  vithout  ruestion  later  than  the  garnet,  vesuv- 
ianite,  diopside,  v/ollastonite  and  tremolite,  and  are  more 

1.   Loc.  cit.,pp.  25-37. 


166 


nearly  contemporaneous  v;lth  the  epidote,  chlorite  and  serioite. 
The  zeolites  (laumontite  and  heulandite)  and  the  calcite  in 
part  are  later  than  the  sulphides.   The  occurrence  of  the 
zeolites  in  association  with  hi£:h  temperature  contact  - 
metamorphic  ninerals  is  interesting:  testimony  of  the  change 
from  intense  conditions  of  temperature  in  the  early  stages 
of  the  mineralization  to  mild  hydrothermal  conditions 
toward  the  close. 


Ho.   8    Lerel 
6 BO  feet 


'/& 


No.  II    Level 
-feet 


'inze 

.  10  Level 


Sfaft 


Sc-A? 

o    so'    too' 


Crystalline.  Limestone   repbceJ                     Ore 

limestone.  by    contact     silicates                  stapes 

Fig. 10,  Plans  of  ore-bodies  on  different  levels  of 

the  Ifirble  Bay  Mine,  Texada  Island.1 


1.   R.  C.  MoConnell,   Loc.  cit. 


'i 


167 


Diagram  of  Mineral  Sequence 


Intense 


Mild 


Oxidation 


Vesuvianite 

Garnet 

Diopeide 

Cordierite 

Titanite 

Wollastonite 

Trenollte 

Zpidote 

Chlorite 

Serioite 

Quartz 

Calclte 

Pyrite 

Chalcopyrlte 

Sphalerite 

Pornlte 

Galena 
(  Stelnrnannlte ) 

Tetrahedrite 
Klaprotholite 
Launontite 
Heulandite 

Molybdenite 

Chalooolte 

Covellite 

Azurite 

Malachite 


\ 


r 


168, 


Pyrite  at;  usual  io  the  earliest  sulphide. 
chalcopyrite  e.nd  borr.ite  are  represented  to  be  of  essentially 
contemporaneous  origin,  "out  by  the  form  of  the  curves,  it  is 

rented  that  in  the  early  phases,  the  formation  of  the 
chalco. -,  'ito  preco  ruinate  a,  while  in  the  later,  the  formation 
of  bornito  ;ti.  i.iore  important.  Sphalerite  accompanied  the 
chal copy rite  i-.\  its  period  of  greatest  development;  galena 
&nd  tetrahedritg  acooaip&nied  the  bornite.  Tho  fclaprotholite 
is  somewhat  luter  than  most  of  the  "bornito. 

•)rigi£j  of  the  seoor.dary  eulphidos*  The  ooourrenoes  of 
chaloooite  eni3  c'ovellite  even  in  am&ll  t^ounts  as  rsplaoements 
of  bornite  at  a  depth  of  about  1000  ft.  bolow  sea-level  is 
surprising.  The  minerals  are  In  forms  characteristic  of  the 
zor.e  of  secondary  sulphides,  In  the  speoinaen  in  whioh  the 
ohaloosite  is  moet  &bund^.nt,  azurite  alao  occurs,  so  there  oan 
be  no  doubt  that  oxidizing  conditions  had  penetrated  to  this 
depth •  The  rocks  ho-.vever  are  tight  and  fairly  dry,  and 
although  there  ie  flo.v  nlong  certain  prominent  fractures  as 

ntioned  previously,  there  iu  practically  no  movement  of 
water  through  tho  rooke.  Aocordin£.  to  MoGonnall,  the  most 
recent  change  vith  relation  to  the  soa-level  ,vas  uplift,  so  a 
higher  position  in  tho  iuuieJiato  past  can  not  be  postulated. 
In  pro— ;••!;:•  oifil  times  the  present  ore  ,vus  undoubtedly  more 
do       •    for  thare  is  abundant  eviuenco  that  glacial 
orosior.  in  thi;-  re^jo,.       ry  r.evore* 


169, 


nevertheless  in  spite  of  these  unfavorable  conditions 
the  ohalooolte  is  there,  and  in  forms  which  impel  bolief  that 
it  is  secondary* 

faters  may  retain  their  oxidizing  powers  even  after  peno- 
trating  to  great  depths  in  barren  limestone,  for  the  rook 
contains  few  minerals  susceptible  to  chemical  alteration 
under  oxidi",inp  conditions.  It  is  possible  that  slow  lateral 
migration  of  waters  of  this  sort  may  have  produced  the  slight 
readjustments  indicated  by  the  chalcocite  and  oovelllte  in 
the  "bornlte.  The  azurlta  associated  with  the  ohalcoclte  may  b 
formed  by  mildly  o;:ldi-in;T  solutions  rich  in  carbonate  ions 
acting  on  bornite  and  the  excess  of  copper  from  the  reaction 
would  be  available  for  the  production  of  ohaloocite  and 
oovellite  in  cracks  beyond  the  reach  of  the  feeble  oxidation. 
The  tightness  of  the  rooks,  and  the  relation  of  the  deposit 
to  present  and  past  aea-levels  and  to  older  topography  are 
all  unfavorable  for  the  extension  of  secondary  changes  to 
notable  depths.  The  occurrence  of  small  amounts  of  asurite, 
ohalooclte  and  oovellite  so  deep  below  sea-level  indicates 
the  grebt  ease  with  which  the  alteration  of  bornite  to 
secondary  sulphides  takes  place. 

Summary.   The  ores  of  the  Marble  Bay  Mine  are  of 
oonteot-metamorphic  origin,  and  occur  in  limestone  near 
intrusions  of  diorite  porphyry.  The  sulphides  replaoe  the 
llmeetone  and  earlier  hifrh  temperature  silicates.  They 
are  closely  associate*'  .;ith  epidote,  chlorite  and  serioite, 


'it 

. 

'•• 

' 

.  6J-.' 

' 


' 


170, 


The  rich  ore  "bodies  are  usually  situate^  between  garnetized 
limestone  on  one  side,  and  pure  marmorized  limestone  on  the 
other.  The  sulphides  are  "believed  to  have  been  'ormed  under 
conditions  of  intermediate  intensity,  .milder  than  those 
.vhloh  produced  the  gurnet,  ve^uvienite  and  diop^lde,  "but 
more  intense  than  those  .viii'Xi  .produced  the  zeolites. 

Bornite  is  the  chief  oro-naineral,  but  ohalcopyrite  is 
also  abundant.  The  two  minerals  are  olosely  associated  for 
the  most  part  but  the  bornite  is  slightly  later*  A  mineral 
doubtfully  identified  ao  fclaprotholite  was  observed  as  a 
replacement  of  bornite.   ?jrite  ie  preotioally  absent  in 
the  ores  in  the  altered  limestone. 

The  great-  ease  of  the  alteration  of  bornite  to 
secondary  sulphides  is  sho.'/n  by  the  development  of  a  small 
amount  of  chaloocite  and  oovellito  on  the  10th.  level, 
under  conditions  particularly  unfavorable  for  deep  enrichment. 


171, 


Blebeo.  Arizona. 

The  time  has  not  been  available  either  in  the  field 
or  in  the  laboratory  for  ae  thorough  a  study  of  the  complex 
problems  connected  with  the  Bisbee  ore-deposits  as  has  been 

given  many  of  the  other  camps.  The  abundance  of  reliable 

1 
information  in  accessible  form,  however,  makes  It  unnecessary  to 

do  more  than  merely  outline  the  most  Important  features  re- 
lated to  the  distribution  of  the  bornite  in  the  primary  ores 

1.  James  Douglas,  The  Copper  Queen  Kin®,  Tran.  A.I.M.E., 

Vol.  29,  PP.  511-51-6,  (1900). 

P.  L.  Raneone;  Copper  deposits  of  Bisbee,  Ariz.,  U.3.(J.3.f 

Bull.  213,  1903,  pp.  14-9-157. 

The  geology  and  ore  deposits  of  the 
Bisbee  Quadrangle,  Ariz.,  U.S. a. 8.,  Prof. 
Paper  21,  1904-. 
Bisbee  folio,  U.S.O.S.,  No.  112,  (190M-). 

Weed,  w.  H..   The  copper  mines  of  the  United  States  in 

1905:   U.S.O.S.,  Bull.  233,  1906,  pp. 99-100. 

Arthur  Notinan,  The  Copper  Queen  nines  and  Works,  Part  II, 

Trans.  Inst.  Mining  and  Metallurgy,  Vol.  22, 
PP.  550-562,  (  1913  )• 

W.  L.  Tovote,  Bisbee,  a  geological  sketch,  Mining  and 

Scientific  Press,  vol.  102,  pp.  203-20K, 
(Feb.  1911). 

Y.  S.  Bonlllas,  J.  B.  Tenney  and  Leon  Pouchere, 

Geology  of  the  Warren  Mining  District, 
Trans.  A.I.M.E,  (Bull.  117,  pp.  1397- 
14-65;  Sept.  1916).  A  detailed  description 
of  the  ore-bodies,  based  on  intimate 
knowledge  of  both  the  field  and  micro- 
scopic relations. 


• 


oK  iw 


172. 


and  the  character  of  Its  alteration  to  chalcocite.  ;jy  in- 
formation of  the  field  relations  was  gained  largely  from 
iiesBrs.  Y.  S.  Bonillae  find  Leon  Feuchere  of  the  Copper  Queen  Co. 
and  Mr.  Ira  Joralsmon  of  the  Calumet  and  Arizona  Co.  during 
the  five  days  spent  with  them  in  the  district. 

Primary  features  of  the  bornite  ores.  Bornite  is 
found  in  all  parts  of  the  Biabee  ores,  but  in  general  it  is 
lees  abundant  than  ohaloopyrite.   In  the  disseminated  ores  of 
Sacramento  Hill,  however,  ohiloopyrite  is  lacking  and  the 
primary  ore  consists  only  of  pyrite  and  bornite.  The  porphyry 
in  which  the  bornite  is  associated  is  serloitlzed  In  general, 
and  locally  silioified.  Elsewhere,  as  in  the  contact- 
raetaiaorphio  zone  or  in  many  of  the  ore-bodlee  in  relatively 
unaltered  limestone,  chalcopyrlte  predominates  although 
bornite  is  usually  present.   High  temperature  silicates  such 
as  garnet,  dlopside  and  wollastonite  occur  in  the  contact 
zone,  but  only  in  snail  amounts.  The  development  of  pyrite 
is  believed  to  be  later  than  these  minerals,  and  closely 
associated  with  the  sericitlzatlon  of  the  porphyry  and  sed- 
iments, while  the  chalcopyrite  and  bornite  are  somewhat  later 
ani  accompany  the  silicification.  The  relations  indicate  that 
ohilcopyrite  and  bornite  were  produced  under  the  same  con- 
ditions, but  that  the  formation  of  bornite  lagged  somewhat, 
and  in  part  was  a  little  later  than  the  chalcopyrite.  Sphaler- 
ite in  small  ^uounts  is  usually  associated  with  chaloopyrlte 
v:hile  galena  is  later 
°nx  related  more  closely  to  the  development  of  the  bornite. 


173. 


Secondary  alteration.  The  distribution  of  oxidation 

and  enrichment  la  well  described  and  Illustrated  in  the  recent 

1 
literature.    Oxidation  and  the  development  of  chalcooite  are 

deepest  in  the  ores  in  little  altered  limestone,  although 
in  these  cases  an  actual  reduction  of  values  per  unit  vol- 
ume ins  taken  place  rather  than  an  enrichment.  In  the  serlclt- 
ized  porphyry  ores,  oxidation  is  slower  and  a  distinct  zone 
of  sulphide  enrichment  is  found.  In  the  silicified  contact 
breccia,  where  the  ores  are  least  permeable,  the  zone  of  ox- 
idation and  secondary  sulphides  is  relatively  thin. 

2 
According  tc  Bonlllas,  Tenney  and  Feuehdre,  the 

mine  workings  have  now  penetrated  to  a  depth  at  which  the 
rocks  as  well  as  the  ores  show  no  effects  of  surface  changes, 
iul  continue  downward  without  the  vertical  variations  oom- 
non  in  the  upper  portions.  "Chalcocite  is  very  abundant  in 
many  forms  in  the  zone  of  enrichment  but  has  not  been  seen 
in  the  undoubted  primary  zone,  though  this  has  been  consider- 
ably explored. "  Consequently  the  geologints  of  the  Copper 
Queen  Company  conclude  that  it  is  highly  probable  that  the 
ohaloocite  was  not  formed  by  ascending  solutions  in  this  dis- 
trict. The  force  of  this  conclusion  is  slightly  weakened  by 
the  fact  that  the  zone  of  enrichment  is  so  deep  that  primary 
variations  in  the  ore  could  conceivably  be  confined  to  it. 
The  greater  abundance  of  galena  on  the  upper  levels  offers  an 
example  of  such  a  change,  for  this  mineral  is  surely  of 
primary  origin. 

1.  Y.  S.  Bonillas,  J.  B.  Tenney,  and  Leon  Feuchere. 
Loo.  oit. 


174, 


The  chalcoolte  Is  to  a  large  extent  a  replacement  of 
bornite.  The  ratio  of  pyrite  to  chalooclte  in  many  of  the 
enriched  oros  is  approximately  the  sane  as  the  ratio  of  pyrite 
to  bornit©  in  the  primary  ores,  and  there  can  be  little  doubt 
that  In  these  the  pyrite  remained  fairly  resistant  to  the  en- 
riching processes.   However,  in  some  of  the  richer  ohal- 
cooite  ores,  especially  those  mined  some  years  ago,  enrich- 
ment was  sufficiently  intense  to  have  largely  replaced  the 
pyrite  as  well  as  the  other  sulphides.  Chaloopyrite,  as 
usual,  alters  less  readily  than  the  bornite,  but  in  many 
parts  of  the  ores  it  has  been  converted  completely  to  ohaloo- 
cite.   Chalcocite  has  been  observed  associated  with  bornite 
in  blebs  in  galena,  which  suggests  that  the  latter  is  more 
resistant  to  chalcocite  alteration  than  the  former. 

Jiicroscopical  study  of  the  bornite-ohalcocite  ores, 
however,  reveals  a  great  variety  of  relations  between  the  two 
minerals.  In  most  cases  veinlets  or  rims  afford  clear  ev- 
idence that  the  chalcocite  in  part  at  least  is  of  replacement 
origin.  In  such  relations,  the  distribution  of  the  chalco- 
clte  if*  clearly  dependent  on  obvious  channel-ways,  such  as  the 
surface  of  contact  between  sulphide  and  gangue  minerals,  or 
along  graifc  boundaries  or  definite  fractures  in  the  bornite. 
In  the  deeper  ores,  or  in  places  Where  the  development  of 
chalcocite  is  less  intense,  the  alteration  of  bornite  yields 
the  lattice-structure.  In  it  all  degrees  of  replacement  are 
shown  from  the  initial  strips  of  chalcocite  penetrating 
bornite  fields  to  the  final  complete  stages  where  bornite 


175. 


remains  only  In  bluish  residues  in  broad  areas  of  chaico- 
cite.  The  triangular  or  rhombic  pattern  of  the  bornlte 
chalcocite  lattice  is  inherited  by  the  chalooeite.  It  is 
shown  either  in  the  blue  and  white  tints  of  the  polished  sur- 
surface  or  by  the  etch-patterns  developed  with  nitrio  acid  or 
potassium  cyanide.  The  white  spines,  which  correspond  to 
the  first  invasions  of  the  chalcocite  into  the  bornite  etch 
more  readily,  while  the  blue  areas  between  then  are  more 
resistant,  and  the  lattice  pattern  is  thus  emphasized. 
Certain  of  the  linss  produced  by  etching,  however,  are  not 
necessarily  parallel  to  the  lattice  directions.  The  blue 
patches  frequently  etch  with  only  one  set  of  parallel  lines, 
usually  making  a  small  angle  to  one  of  the  directions  of  the 
lattice  pattern. 

The  replacement  origin  of  the  chaloocite  in  the 
lattice-structure  can  not  be  doubted,  but  its  attack  on  the 
bornlte  takes  place  in  a  selective  manner  which  is  difficult 
to  explain.   Certain  grains  or  parts  of  grains  nay  be  largely 
reduced  to  rhombic  or  triangular  residues  of  bornite  between 
Intersecting  bands  of  chalcocite,  while  the  adjacent  bornite 
maintains  a  Rmooth  resistant  boundary  against  the  corroding 
agents.  The  distribution  of  chaicocite  in  the  lattice 
structure  does  not  show  the  same  dependence  on  obvious  channel- 
ways  for  solutions  such  as  grain  boundaries  and  fractures 
that  is  commonly  exhibited  by  secondary  sulphides. 

The  graphic  structure  between  bornite  and  chalcocite 
is  only  slightly  developed  in  the  Bistoee  ores,  but  irregular 


176. 


blebs  and  specks  of  bornite  scattered  through  chalcocit« 
fields  often  assume  shapes  very  similar  to  It  and  are  common. 
In  modified  form  it  has  been  observed  associated  with  the 
lattice  pattern,  in  relations  which  suggest  that  the  chalcocite 
between  tho  irregular  bornite  blebs  was  derived  by  replace- 
ment.  Numerous  small  grains  of  bornite,  with  sharp  clean 
outlines,  often  remain    mattered  through  fields  of  ohalcocite 
which  are  clearly  products  of   lattice  replacements.  The 
distribution  of  these  grains  in  many  cases  greatly  resembles 
certain  types  of  graphic  patterns.  The  bornite  grains  are 
Clearly  isolated  residues  in  chaloooite  of  replacement  origin. 
It  must  be  noted,  however,  that  the  lattice  replacement  in 
the  final  stages  normally  does  not  yield  products  of  this 
sort,  but  sub-angular  patches  of  bornite  with  hazy  boundaries. 
The  discussion  of  these  significant  relations  will  be  postponed 
until  the  subject  can  be  treated  in  a  more  general  way  on 
later  pages. 

Bornite  residues  or  inclusions  in  chaloocite  assume 
various  peculiar  forms.  Bands  of  bornite  near  the  margins  of 
ohalcoolte  areas  are  not  uncommon.  ( Fig.    ).   Spines  of 
bornite  remain  apparently  resistant  in  areas  of  well  advanced 
lattice  replacements.  In  other  places,  bornite  occurs  as  a 
fine  peppery  sprinkling  of  aharply-^ef ined  specks  in  chal- 
cocite.  This  association  has  been  termed  by  J.  Murdoch  the 
"drop-erjulsion  structure,"  which  is  a  very  descriptive  name, 


177, 


but  as  the  term  "emulsion"  suggests  a  contemporary  origin 
for  the  two  minerals,  a  lees  definite  name,  such  as  "drop- 
structure11  is  perhaps  to  be  preferred. 

Fine  bornite  stringers  in  coarse  caloite  veins  have 
been  observed  almost  completely  replaced  by  chalcoclte, 
which  indicates  that  the  enrichment  of  bornite  to  chalcocite 
is  possible  in  the  presence  of  calcium  carbonate,  and  that 
the  acid  liberated  by  the  reaction  is  not  sufficient  to  cor- 
rode the  limestone  to  any  notable  extent. 

Secondary  bornite  is  uncommon,  although  a  snail 
amount  exists  in  the  Bisbee  ores.  Fine  veinlets  of  chalcocite 
cutting  bornite  areas  have  been  observed  to  change  into  vein- 
lets  of  bornite  when  chalcopyrlte  was  encountered.  Chaloo— 
pyrite  residues  in  chalcoclte  in  many  cases  possess  complete 
or  intermittent  rims  of  bornite,  which  suggests  that  the 
alteration  of  chalcopyrite  to  chalcoclte  may  taJce  place  through 
bornite.  A  more  complete  sequence  la  shown  by  successive  rims 
of  chalcopyrite  and  bornite,  which  may  be  termed  "haloe", 
around  pyrite  grains  in  a  chalcoclte  field.  The  secondary 
origin  of  the  bornite  in  these  cases  IB  probable,  but  the 
quantity  of  the  mineral  involved  is  very  small.  (  Fig.  153 ). 

Suimary.  The  bornits  at  Bisbee  is  a  replacement  of 
porphyry  or  limestone  under  milder  conditions  than  those  under 
which  the  contact  metamorphic  silicates,  the  pyrite,  or  the 
sericite  were  developed.  The  bornite  is  accompanied  by  sil- 


178. 


icification  usually,  but  in  Boni?  places  replaces  limestone 
v;ithout  the  development  of  other  minerals .  It  occurs  both 
in  pyritio  ores  and  in  those  poor  in  pyrite. 

Froir.  the  fiold  evidence  it  is  probable  that  the 
ehaloocite  associated  with  the  bornite  is  all  of  secondary 
origin.   The  microscopical  evidence  indicates  that  the 
chalcocite  is  to  a  large  extent  a  replacement  of  bornite.  The 
chalcocite  in  the  lattice  structure  apparently  shows  a  se- 
lect tive  action  in  its  attack  on  the  bornite,  and  there  is 
evidence  vrhioh  suggests  that  the  chalcoclte-bornite  graphic 
structure  nay  be  produced  by  a  somewhat  similar  selective 
replacement. 


179. 


DEPOSITS  OF  HYDRO-THERMAL  ORIGIN 
The  Matnsa  Mine.  Superior.  Final  Co..  Arizona 

INTRODUCTION 

The  Magma  Mine  is  worthy  of  particular  attention 
in  connection  with  the  study  of  th?  problems  associated 
with  bornite  for  a  large  percentage  of  its  cojper  output 
is  derived  frost  this  mineral.  From  a  commercial  standpoint, 
it  is  at  present  the  most  important  bornite  deposit  in  the 
United  States,  with  the  exception  of  certain  ore-bodies  at 
Butte  and  Bisbee. 

Situation.  The  Pioneer  Mining  District  of  Final 
Co.  is  situated  about  65  miles  east  of  Phoenix,  in  the  re- 
gion of  indefinite  rugged  ridges  and  peaks  south  of  the  Salt 
River.  The  chief  town  in  the  district  is  Superior,  which 
is  supported  almost  entirely  by  the  Queen  Mine  of  the  Magma 
Copper  Company.  The  actively  producing  camp  of  Ray  is  only 
16  miles  to  the  east,  but  across  a  rough  ridge.  A  narrow 
gauge  railroad  extends  to  the  south,  connecting  with  the 
Arizona  and  Eastern  Railroad  near  Florence.   In  addition  to 
the  railroad,  an  automobile  stage  service  between  Ray  and 
Phoenix  passes  through  Superior  and  makes  the  camp  readily 
accessible. 

The  town  of  Superior  is  built  on  a  gentle  slope  of 


. 

JIOZl 

- 

-*     . 


. 

ag^i  atfjtn  lo  o 

.   leiric   silT     .Tff- 

:i    fc»t". 

.  :    .. 


»  Q 

i 

I 


180. 


unconsoli  dated  desert  gravels  on  the  northern  aide  of  an 
irregular  structural  valley,  drained  chiefly  by  Queen  Creek. 
Immediately  to  the  east,  an  Imposing  precipitous  ridge  known 
as  Apache  Leap  rises  about  1500  feet  above  the  town,  and  pre- 
sents to  the  geologist  a  fascinating  section  of  the  rooks  of 
the  district,  exposed  with  almost  diagramatio  clearness. 
Queen  Creek,  which  fringes  the  town  on  the  south,  breaks  the 
escarpment  with  a  deep  gorge,  along  which  the  eastward! y  dip- 
ping strata  are  out  in  succession,  as  one  ascends  the  stream. 

Mines.  The  only  ore -body  in  the  vicinity  of  Su- 
perior which  supports  a  r.ine  of  importance  ia  property  of  the 
Magma  Copper  Company.   The  ore  is  almost  entirely  confined 
to  a  porphyry  dike,  which  varies  from  a  few  feet  to  over 
40  feet  in  width.   It  is  worked  by  a  1500  foot  shaft,  and 
levels  at  the  usual  intervals.  At  the  time  of  my  visit, 
(August,  1S16),  the  deepest  workings  were  on  the  1200  level, 
but  since  then  the  main  cross  cut  from  the  shaft  at  a  depth 
of  1500  feet;  has  encountered  the  dike  and  found  good  bornite 
ore.  The  ores  are  rather  complex,  and  in  the  deeper  ore- 
bodies  silver,  lead  ani  zinc  values  are  sufficient  to  make 
their  recovery  worth  vrhila.  Flotation  has  proved  to  be 
the  most  satisfactory  method  of  concentration. 

The  Silver  King  Mine,  five  miles  to  the  north,  has 
been  a  famous  silver  producer  in  the  past  but  it  has  not  been 
in  operation  for  many  years.  The  ruins  of  the  old  mill, 
surrounded  by  crumbling  adobe  walls  of  the  small  town  it 


..'  38. 


181. 


supported,  may  be  seen  from  the  Phoenix  stage  road,  ten  miles 
or  so  east  of  Superior,  anl  still  testify  to  the  former  great- 
ness of  the  property.   The  Lake  Superior  and  Arizona  Mine, 
immediately  north  of  the  town  of  Superior  possesses  exten- 
sive drifts  along  the  strike  of  the  upper  surface  of  the 
Cambrian  (?)  quartzite,  but  it  has  developed  little  ore. 

Literature.   Some  notes  on  the  geology  and  the  ore- 

1 

bodies  of  the  district  have  been  published  by  F.  L.  Raneome, 

& 

but  no  detailed  paper  has  yet  appeared.  Earlier  mine  reports 

furnish  a  little  information,  ohiefly  of  value  for  the  de- 
scription of  the  older  and  now  exhausted  silver  ores.  An  ex- 
cellent topographic  and  geologic  map  of  the  ridges  near  the 
mine  has  been  oa.de  by  the  engineers  of  the  Magma  Copper  Co., 
but  the  v?ork  has  not  been  published. 

Acknowledgements.  My  thanks  are  lue  to  Mr.  W.  C. 
Browning,  the  superintendent,  for  the  privilege,  of  visiting 
the  mine,  and  for  several  rare  specimens.  A  large  part  of 
my  information  concerning  features  of  geological  interest 
in  the  mine  and  in  the  immediate  neighborhood  was  obtained 
from  Mr.  I.  A.  Ettlinger,  the  ohief  engineer,  who  very 
kindly  devoted  many  hours  to  showing  roe  all  important  parts 
of  the  deposit. 


1.  Bulletin  540  D.  U.S.G.S.,  Copper  Deposits  near 
Super ior,Ari 20.  ,    (1913), 

3.  One  on  the  Silver  King  Mine,  quoted  by  Ransome,  is 
by  W.  P.  Blake,   Description  of  the  Silver  King  Mine  of 
Arizona,  New  Haven,  1838,  48  ,p.  with  illustrations. 


181A. 


.A  13 1 


PLATE  V 
Super i  of  |  'rizcna. 


11  View  to  the  south  from  the  Magma  Mine. 
C«KrlM(T)  quartzite  outcrops  on  he  lower 
slopes,  with  Carton! feroua  limestone  above, 
capped  by  the  great  daoite  oliffs  of  Apache 
Leap. 

12  Aoaohe  Leap  from  the  west.  The  town  of  Su- 
SSrior  at  the  base  of  the  oliffs  may  be  seen 
on  the  right.  The  deposit  of  the  1  :agma  Mine 
is  situated  along  the  fault  shown  by  the  dis- 
placement of  the "white  limestone  beds  juat  to 
the  left  of  centre. 


181B. 


PLATE  VI 
Superior,  Arizona^ 

Fig.  in.  The  Magma  Mine  from  the  south. 


Fl?.  1-'..  Outcrop  of  the  Magma  deposit.  The  prominent 
cropping  in  the  foreground  ia  quartzite. 


isa. 


General  Geology 

Sedimentary  Column.   The  geologic  column  is  ex- 
posed with  satisfactory  thoroughness  on  the  bcld  cliffs  of 
Apaohe  Leap,  -And  in  the  canon  dissecting  it.   The  lowest  of 
the  sedimentary  beds,  and  the  oldest  rocks  exposed,  are 
massive  quartzite a,  averaging  about  200  feat  in  thickness. 
They  are  of  pro -Devonian  age,  and  are  probably  to  be  cor- 
related with  the  Cairbrian(?)  formations  described  in  the 

1 

neighboring  Globe  and  Ray  Districts.   The  quartzite  is  over- 
lain conformably  by  a  thousand  feet  or  more  of  limestone, 
ranging  froir.  Devonian  into  the  Carboniferous.  There  are 
several  well  marked  variations  in  the  character  of  the  lime- 
stone, which  have  enabled  the  local  geologists  to  irake  seven 
or  more  subdivisions  of  the  series. 

Igneous  Rooks.  The  igneous  rocks  noted  are  diabase, 
monzonite.  porphyry,  and  iacite.  Thi  diabase  forms  large 
sills,  the  only  one  of  importance  at  the  mine  being  intruded 
into  the  quartzite.   It  is  at  least  1100  feet  thick,  as 
shown  by  the  rrdr.s  workings,  and  the  floor  has  not  been 
reached  as  yet.  There  are  said  to  be  exposures  in  the  neigh- 
borhood, however,  where  the  diabase  sill  may  be  seen  rest- 
ing on  atill  lower  quartzite  beds,  but  the  short  time  avail- 
able did  not  allow  roe  to  see  them.  A  few  irregular  trap 
dikes  in  the  region  are  probably  of  the  same  general  period 


1.  F.  L.  Ransome,   Some  Paleozoic  sections  in  Arizor 
their  correlations,   Prof.  Paper,   38-K,  US.  6.S.,  1916. 


183. 


of  intrusion. 


certon/f^i^  ^ForPhyy  J''fa 

limestone 


ISA.        North -south    section      through        the     Ma3n,a  Mine  . 
Scale    •     I  in    =     IDoo  ft. 


Figure. 15 B 
West-east  section  through  Acache  Leap^  near  Magma  Mine. 


The  porphyry,  whioh  ie  locally  termed  granite 
porphyry,  occurs  in  a  few  dikes,  the  most  important  being 
the  large  dike  in  whioh  the  ores  of  the  Queen  Mine  occur. 
The  rook  has  a  light  gray  color,  with  a  slightly  greenish 
tinge.  Feldspar  phenooryats  ooour  sparsely  scattered  in 
an  aphanitio  ground  case.  Under  the  microscope,  the  orig- 
inal constituents  in  the  specimens  studied  were  found  to  be 
all  greatly  altered  by  the  mineralizing  processes,  but  the 


184. 

original  character  of  the  rock  may  be  reconstructed  with 
little  doubt.  The  phenocrysts  are  chiefly  orthooiase,  acid 
plagioolase,  and  thin  laths. of  hornblende.  The  ground  mass 
ie  miorogranitio,  and  although  now  highly  silioified  it 
probably  contained  little  quartz  originally.  The  rook,  aa 
far  as  may  be  inferred  from  microscopical  study,  is  more 
properly  termed  a  monzonite  porphyry  than  a  granite  porphyry* 
Variations  in  the  petrographic  character  of  the  dike  are  said 
to  occur,  however,  Mr.  Fred  B.  Ely,  an  engineer  of  the  com- 
pany, believes  that  the  dike  it  composite  in  character  but 
during  my  short  time  at  the  mine  I  saw  only  one  type  of  in- 
trusive. Ransome  states  that  the  rook  is  probably  a  quarts 
diorite  porphyry,  but  adds  that  the  determination  is  not  cer- 
tain on  account  of  its  greatly  altered  condition.  It  is 
quite  possible  that  there  is  a  notable  variation  in  the  char- 
acter of  the  rook  but  several  specimens  collected  from  dif- 
ferent parts  of  the  mine  all  indicate  a  monzonite  composition 
for  the  chief  mass  of  the  dike. 

The  most  prominent  igneous  rook  in  the  vicinity  of 
Superior  is  daoite,  which  occurs  as  thick  flows  resting  on 
a  fairly  even  erosional  surface  out  roughly  parallel  to  the 
bedding  of  the  Carboniferous  limestone.  The  rook  is  of 
Tertiary  age,  and  later  than  the  dikes  previously  described. 
The  Tertiary  sediments  which  occur  with  the  lavas  in  the 
Globe  and  Hay  Districts  are  not  exposed  in  the  immediate 
vicinity  of  the  mine.  Coarse  desert  wash  slightly  cemented 


"io 


185. 


fan glomerate,   fills  the  valley  to  the   south  to  an  indefinite 
thickness,   and  nearly  completely  mantles  the  older  rooks. 

Structure.     The   sedimentary  beds  and  the  parallel 
igneous  formations  strike  roughly  north  and  south,   and  dip 
30-40     to  the  east.     The   deformation  is  chiefly  due  to  the 
faulting  of  post-iaoite  age.      In  the  vicinity  of  Superior, 
the  beds  are  broken  by  a  strong  north-south  fault,   which 
extends  along  the  base   of  the   steep  eastern  slope  of  Apache 
Leap.     The  boldness  of  the  escarpment  testifies  to  the 


\ 

\ 

\ 

^ 

g 

1 

•^ 

• 

1 

^  ' 

"1  *- 

\                    Carkoni  ferous 

S 

Carboniferous                                Dae  ite. 

\                                limestone 

w 

P5* 

linestone 

N 

^.  »-^ 

\ 

(/t 

**t  ^^ 

h 

S; 

ft 

\-so- 

i 

^V 

£ 

1 
\ 

\ 

\ 

fv, 

1 

*~  *~  ^ 

^ 

\ 

I* 

\^    Ddcitc. 

3 

Carboni  fereiis 

Da  cite. 

\ 

1 

lime,  stone 

\ 

<s. 

\ 

n 

Wash 

'    ' 

\ 

(» 

s 

h 

s 

Scale   :        /"=    /ooo'        (appro*. 
Sketch      mere/if       aiaorammatic 

•J  -J 


Figure  , 


Sketch  map  of  geology  near  Superior.  Arizona. 


of 


.31, 


186. 

youthfulnesa   of  the  movement,    arid  its  height  etill  approxi- 
mately indicates  the   throw.     Immediately  to  the  west  of 
the  fault  near  the   Queen  Minu,    daoite  out-crops  through  the 
wash  of  the  valley;    to  the  east,    the   thick  flows  of\  daoite 
form  the  even  ore at  of  the  ridge,   and  rise   as  a  line  of 
great  splintery  cliffs  over  500  feet   in  height     above   the 
gentler  slopes  of  the  limestones.     Near  Superior,    the  ac- 
tual break  is  oonoealed  by  the  desert  wash,   but  north  of 
the  nine,   a  spur  of  the  ridge  extends  to  the  west  of  the 
fault,   due   to  complications  caused  by  cross  fractures.     The 
displacement  near  the   town  is  over  1000  feet,  but  beyond 
the  mine,    it  becomes  less,   and  the  fault  passes  into  a  fold, 
which  gradually  dies  out  to  the  north. 

The  ore-bearing  porphyry  dike  at  the  Magma  Mine 
is  intruded  along  an  east-west  fault.     The   fault  is  older 
than  the  north-south  fault  just  described,   and  is  offset  by 
it.     The   dike  and  ore a  do  not  extend  into  the  daoite;    this 
suggests  that   the  break  originated  in  pre-dacite   times.     The 
daoite,   however,    is  displaced  along  the   line  of  the  fault, 
which  indicates  a  renewal  of  movement  along  the  old  plane 
of  weakness.      The   total  displacement   is  about  600  feet,   with 
the   down-throw  side   to  the  south.     The   dip  of  the  fault  and 
the  dike  at  the  Queen  Mine  is  about  70°   to  the  north  on  the 
upper  levels,   but  below  the   900  feet  level  it  reverses  to 
to  about  70-80°   to  the   south. 

There  are   several  other  smaller  breaks,    some  of 


OOOO    el    JUjttT 


z 


187. 
which  offset  the  ore  slightly,  but  none  are  of  any  magnitude. 


Primary  Mineralization. 

Form  of  Ore-Bcly.  The  ore  at  the  Queen  Mine,  as 
has  been  mentioned  previously,  is  largely  confined  to  the 
porphyry  dike  intruded  along  of  the  east-west  fault  surface. 
The  dike  has  a  maximum  width  of  40-50  feet,  but  in  places 
it  pinchee  to  a  thin  seam,  or  gives  way  entirely  to  a 
slickensided  unmineralized  fracture.  The  mineralization  is 
most  intense  in  the  widest  portions  of  the  dike,  and  grows 
weaker  where  the  dike  becomes  narrow.  It  usually  ends  many 
feet  before  the  dike  pinches  completely*  The  sulphides  occur 
in  seams  ranging  from  a  fraction  of  an  inch  up  to  massive 
bodies  several  feet  in  thickness,  or  as  disseminations  in  the 
porphyry.  The  former  are  by  far  the  most  important.  One  ex- 
ception to  the  common  relations  between  porphyry  and  ore  is 
found  on  the  1200  level,  where  a  drift  to  the  south  inter- 
sects a  vein  of  pure  chaloopyrite,  one  to  two  feet  in  width, 
which  occurs  in  the  wall  rook  many  feet  from  the  dike. 

Distribution  of  Ore-Minerals.  The  distribution  of 
the  primary  ores  shows  little  dependence  on  the  nature  of 
the  wall  rooks.  Ransome  observed  a  tendency  for  the  bornite 
ore-bodies  to  be  richer  along  the  contacts  of  the  dike  and 
quartzite  beds,  but  subsequent  development  indicates  that 
this  relationship  is  not  aa  important  as  it  may  have  ap- 
peared at  first.  The  mineralization  along  the  dike  is  fair- 


:i-;8    flCta    i 
.A    Otf    0o 


*®Cf    $*•! 

.»vae  seiJbod 


>oo 

. 


.cila   aa^o 

- 

- 


133. 


ly  continuous,  gradually  dying  out  to  the  wast,  however,  be- 
fore the  large  north-south  fault  ia  intersected.  In  a  broad 
way  there  seem  to  be  two  vertical  zones  of  somewhat  richer 
ore,  one  to  the  east  of  the  shaft,  the  other  slightly  to  the 
vreat  of  the  shaft.  In  the  neighborhood  of  the  500  feet  level, 
very  rich  bodies  of  massive  ohaloooite  occur  in  the  eastern 

part  of  the  ore -body  which  pass  with  depth  into  ore  consisting 
predominantly  of  bornite.  On  the  same  level,  the  western 
portion  of  the  ledge  contains  rather  lean  pyritic  material, 
which  changes  into  rich  bornite  ores  on  the  levels  beneath. 
Lead  and  zinc  become  of  greater  importance  on  the 
lower  levels.  Along  one  cross-cut  on  the  1100  feet  level, 
the  entire  width  of  the  dike  is  mineralized,  and  the  assays 
along  its  wall  show  a  notable  banding  of  copper  and  of  lead 
and  zinc  values  parallel  to  the  strike  of  the  dike.  A  rich 
copper  band  follows  the  north  side  of  the  ledge  (the  high 
values  being  due  chiefly  to  bornite).  This  changes  sharply 
with  little  intermixing  of  the  minerals  into  a  band  lean  in 
copper  and  high  in  lead  and  zinc,  which  is  in  turn  succeeded 
by  another  rich  copper  zone,  with  low  lead  and  zinc  values. 
The  silver  tends  to  rise  with  the  copper,  but  it  is  also 
present  with  high  assays  of  lead  and  zinc.  Besides  these 
variations  in  distribution  of  the  metals  across  the  ledge, 
there  is  said  to  be  a  distinct  tendency  for  the  copper  to 
increase  at  the  expense  of  the  lead  and  zinc  toward  the 
west;  toward  the  east  the  zinc  increases  with  decreasing 


.oo  eio  otal  rttqefc  riJl*  ewsq  rfotriw  tbod-erro  & 


. 


139. 


ooppar  values. 

Mineral  Sequence.  The  mineralization  la  clearly 
later  than  the  crystallization  of  the  porphyry,  for  its 
earliest  products  break  the  rook  in  seams,  or  replace  Its 
primary  structures.  Pyrite,  aa  usual,  was  the  first  sulphide 
to  fora,  and  it  occurs  in  largs  Basses  or  scattered  grains. 
It  was  accompanied  by  intense  ailicif ioation  of  the  ground 
mass  of  the  porphyry,  and  by  the  replacement  of  the  feldspar 
aad  hornblende  phenocryats  by  aoricite.  Both  the  pyrite 
bodies  and  the  altered  dike-rode  offer  abundant  evidence  of 
subsequent  fracturing,  which  afforded  channel  ways  for  the 
following  copper  bearing  phases  of  the  mineralization.  The 
pyrite  is  greatly  shattered  in  places  (Figures  93,  94  ), 
and  the  breaks  emphasized  and  widened  by  bornite  or  ohalco- 
pyrite  replacement.  Veinlets  of  quartz  carrying  bornite  or 
ohalcopyrite,  break  the  areas  of  earlier  quartz  and  sericite. 
While  it  is  possible  that  some  of  the  copper  minerals  may 
have  continued  to  form  with  the  main  mass  of  the  bornite  ;md 
chalcopyrite,  the  evidence  of  two  stages,  viz.  one  in  which 
pyrite  dominated,  arid  the  other  in  which  bornite  and  ohaloo- 
pyrite  dominated,  is  very  convincing. 

Under  the  microscope  the  ohalcopyrite  and  bornite 
nay  be  seen  to  be  in  intimate  association.  For  the  most  part, 
they  exhibit  mutual  boundaries  toward  each  other,  but  here 
and  there  the  ohaloopyrite  is  penetrated  by  imperfect  veins 
of  bornite,  or  grains  of  the  ohalcopyrite  appear  as  if  set 


190. 


in  a  oercent  of  bornite.   The  evidence  is  believed  to  indicate 
that  here  as  at  Engels,  Evergreen,  Marble  Bay,  a..i  other 
carr.ps,  the  two  sulphides  formed  under  the  same  conditions 
and  at  about  the  same  time,  but  that  the  ohaloopyrite  slight- 
ly.preceded  the  bornite.  Under  low  magnification,  however, 
fine  lines  of  oaalcopyrite  appear  in  many  specimens,  apparent- 
ly outlining  bornite  grains.   (Figure  92    ).  When  exam- 
ined tuore  carefully,  these  lines  are  found  to  consist  of 
irregular,  somewhat  elongated,  disconnected  blebs,  and  are 
not  distinct,  continuous  veinlets.  It  suggests  that,  as  the 
bornite  crystallized,  an  excess  of  iron  was  concentrated  or 
localized  along  the  contacts  of  the  grains,  which  resulted 
in  the  formation  of  the  iron  rich  sulphide.  In  the  diagram 
of  mineral  sequence,  this  idea  is  shown  more  clearly  perhaps 
than  it  can  be  expressed  here.   In  quantity  bornite  exceeds 
all  other  primary  copper  sulphides,  and  there  is  no  reason 
to  doubt  tnat  for  the  most  part  it  is  a  direct  replacement 
of  the  silicates  or  of  the  pyrite  with  which  it  is  associated. 
The  replacement  of  ohalcopyrite  by  bornite  was  probably  only 
a  feeble  process,  for  the  evidence  that  it  took  place  is  not 
abundant,  and  amount  of  bornite  which  originated  in  this  way 
is  without  doubt  relatively  small. 

Galena  and  sphalerite  become  of  great  importance 
in  certain  parts  of  the  deposit,  as  may  be  inferred  from  the 
preceding  paragraphs  describing  the  distribution  of  the  met- 
als. They  seem  to  be  closely  related  in  age  to  the  bornite 


• 

IQ  a? 

1 

ifcfii 

i  en 


07 

^»^£DjtIia  & 

?.*Qo8  . 
WffA 


14 

•SO 


191. 

and  the  chaloopyrite.  The  galena  and  bornite  are  associated 
in  many  cases,  in  structures  similar  to  the  graphic  structure 
of  bornite  and  ohalcooite,  although  a  greater  variation  in 
the  else  of  the  blebs  ia  often  shown.   (Figures  89,  90  ). 
The  sphalerite  and  ohaloopyrite  also  occur  in  peculiarly  in- 
timate and  fine  grained  structures  which  suggest  an  origin 
by  contemporaneous  intergrowth  for  the  two  minerals.  In 
a  few  cases,  bands  of  sphalerite  ware  observed  in  bornite- 
ohaloopyrite  areas,  but  even  in  these  relations  there  seemed 
to  be  a  little  corrosion  of  the  sphalerite  by  the  bornite. 
The  relations  usually  exhibited  by  small  areas  of  sphalerite 
surrounded  by  bornite  offer  little  or  no  evidence  concern- 
ing sequence.  Where  sphalerite  andigalena  are  abundant,  the 
veins  of  the  latter  in  the  former,  and  the  manner  in  which 
grains  of  sphalerite  are  surrounded  and  corroded  by  galena 
clearly  indicate  that  the  sine  sulphide  was  the  earlier 
mineral.  Elongated  blebs  of  galena  commonly  occur  in 
the  lines  of  ohaloopyrite  blebs  surrounding  bornite  grains, 

i  seem  of  the  same  age  as  the  ohaloopyrite  in  this  form. 
In  general  the  sphalerite  preceded  the  galena  in  time  of  .. 
formation  aad  was  probably  a  product  of  the  same  early 
conditions  as  those  under  which  the  larger  part  of  the 
chalcopyrittt  was  deposited.  The  galena  became  of  impor- 
tance a  little  later,  probably  associated  with  the  abundant 
development  of  bornite,  and  continued  to  form  with  the 
later  ohalcopyrite  in  the  fine  fringes  of  small  blebs  about 


:ia 

• 


do  lo 


132. 

the  bornite  grains. 

The  galena  in  part  is  the  arsenical  variety  known 
as  steinmannite.  In  physical  properties  it  is  identical 
with  the  ordinary  mineral,  but  mioroohemical  tests  on  the 
polished  surface  yield  distinctive  results.  In  some  speci- 
mens, ordinary  galena  occurs  alone  or  apparently  intergrown 
with  the  arsenical  variety. 

Tetrahedrite  is  also  fairly  common  with  the  ores. 
It  is  found  chiefly  in  the  bornite,  as  smooth  grains,  or  as 
lines  of  elongated  blebs,  associated  with  the  bands  of  ohal- 
oopyrite  previously  inentioned.  In  a  few  oases,  it  is  fair- 
ly abundant  and  there  is  a  little  evidence  that  it  is  most 
common  as  a  product  of  the  closing  portion  of  the  bornite 
forming  period.  In  one  case,  the  tetrahedrite  definitely 
cuts  sphalerite,  but  its  relations  to  the  galena  offer 
little  evidence  of  their  relative  sequence.  On  the  1200 
feet  level,  a  silver-bearing  seam  is  picked  up  by  a  long 
drift  to  the  west,  which  extends  well  beyond  the  limits  of 
the  ore  or  the  porphyry.  The  siliceous  material  of  the  seam 
occurs  between  walls  of  serpentinized  diabase,  and  contains 
a  little  pyrite,  spalerite,  galena  (and  steinmannite )  .  tetra- 
hedrite, and  native  silver.  The  tetrahedrite  is  the  most 
abundant,  and  probably  contains  the  chief  silver  values. 
Native  silver  occurs  also  in  the  upper  workings  of  the  mine 
in  oxidized  ground,  but  the  silver  on  the  1300  feet  level 
has  apparently  no  connection  with  superficial  changes,  and 


. 

. 


,i  ai 

• 

. 

*    fti         , 

. 

1*r~  -  -jffejf     , 

. 

:,j8ea  x  -i-iev  .  . 

•35X3    .    .  -J»W    S. 

, 
,ia«   1o   i-i-'Ijaw 

. 


193. 


is  probably  primary.      (Cf  .   Marble  Bay  iline,    Texada  Island, 
native  silver  on  1300  foot  level,   about  1200  feet  below 
sea 


Oxidation  and  Enrichment 

Water  Level.     The  original  water  level  of  the  ttine 
is  stated  by  Ransoae   to  have  been  about  400  feet  below  the 
surface,   presumably  at   the  shaft.     While  the  ores  above   this 
level  were  alaost  completely  of  an  oxidized  character,    the 
lower  limits  of  oxidation  are  far  deeper,   and  seen  to  have 
little  relation  to  the  present  surface  of  the  country  or 
to  the  surface  of  the  ground  ffftttr. 

Oxidized  Ores.     Below  the  650  feet  level,    it  ia 
noticeable   that  oxidized  products  are  encountered  only  in 
the  eastern  portions  of  the   Jedge.     On  the  650  foot  level, 
the  rr.uin  drift  to  the  west  intersects  a  rather  lean  pyritio 
ore-body;    on   the  saute  level,  passing  eastward,   a  rich  cuprite, 
native  copper  and  carbonate  ore-body  is  encountered.     On 
the  725  fPOt  level,   a  lean  sandy  pyritio  body  ie  found  to 
the  west  while   to  the  east,   oxidized  and  secondary  sulphide 
ores  become   of  importance.     On  the  800  foot  level  the  oxi- 
dized ores  occur  again,  but  farther  to  the  east  than  on 
the  level  above.     The   deepest  oxidised  material  thus  far 
encountered  in  the  nine   is  found     in  the  face  of  a  long 
drift  to  the  eastward  on  the   1200  foot  level,   which  ex- 
tends well  under  the   daoite  flows,    that  cover  the  Carbon- 


t  • 


v 


194. 


iferoue  limestone.  The  relation  of  the  bottom  of  the  oxida- 
tion to  the  geologio  structure  is  shown  in  the  sketch  below. 


Fig.  17.   Fketoh  Showing  the  Bottom  of  Oxidation 
and  Its  Relation  to  the  ftruoture. 


It  may  be  mentioned  here  in  passing  that  post- 
mine  oxidation  is  intense  in  nearly  all  parts  of  the  mine. 
The  temperature  and  humidity  are  high,  and  in  all  except  the 
most  recent  drifts*  the  wall  rooks  are  decomposed  and  softened 
to  a  depth  of  several  inches  or  more.  The  pre-aine  character 
of  the  oxidation  described  in  the  preceding  paragraph,  how- 
ever,  is  certain  in  each  case,  both  from  its  lack  of  depend- 
ence on  the  walls  of  the  drifts,  and  from  descriptions  given 
me  by  Mr.  Ett linger,  who  was  familiar  with  the  conditions 
when  the  drifts  in  question  were  first  opened. 

In  the  oxidized  zone,  cuprite  is  the  chief  copper 
mineral,  but  small  amounts  of  malachite  and  azurite  are 
commonly  associated  with  it.  Native  copper  and,  rarely, 
native  silver  occur  in  small  amounts  both  with  the  oxidized 


. 


<a  * 


JV* 


s^ 


195. 

1 
minerals  and  in  the  ehaloooite  beneath.   Wulfenite  oooura 

in  scull  amounts  in  various  plaoea  in  the  upper  workings. 
Good  crystals  were  obtained  from  the  650  foot  level. 

Secondary  Sulphide^,.  At  present,  little  ohaloooite 
is  found  above  the  500  foot  level,  but  old-timers  tell  of 
rioh  bunches  of  this  sulphide  isolated  in  the  oxidized  zone 
above  the  300  foot  level.  In  the  country  east  of  the  shaft, 
a  large  body  of  ohaloooite  ore  extends  from  a  depth  of  about 
500  feet  to  about  750  feet,  although  some  iray  penetrate  a 
little  deeper,  along  fissures,  etc.  The  underlying  ore  is 
rioh  in  bornite,  and  there  oan  be  little  doubt  from  the  field 
relations,  that  the  ohaloooite  is  an  enrichment  product  of 
the  bornite,  caused  by  descending  meteoric  waters.  To  the 
west  of  the  shaft,  the  pyritio  ore-bodies  on  the  upper  levels 
contain  little  chaloooite,  ^.  1  the  underlying  bodies  of  rioh 
bornite  ores,  contain  it  only  as  microscopic  veinlets. 

The  microscope  also  furnishes  evidence  that  the 
rioh  ohalcooite  ores  were  formed  by  a  replacement  of  bornite. 
In  oertain  chips,  bornite  residues  veined  and  rimmed  by 
ohaloooite  furnish  convincing  proof.  The  polished  surface 
of  the  massive  chalcooite,  when  etched  with  dilute  nitric 
acid  or  potassium  cyanide,  breaks  into  very  fine  irregular 


1.  Compare;  Chase  Palmer  u,::d  E.  S.  Bastin,  The  role  of 
certain  metallic  minerals  in  precipitating  silver  and  gold, 
T.  A.  I.  li.  E.,   Vol.  45;  pp.  334-238,    1914. 


JQ&l   t  ftT&w 

,^netQ*xq  ?&  -fo***?  VU&OOOS8 

. 

*BJB»  x^^fl^t  ->d^   ®» 

:.  - 
XJfl 

•a*i  S- 

-    -  •  ~  9TC 

I 

' 

. 

oo«o*s:o.c.a    .,,  vi" 
1  ©tfr 

. 


.mflaf. 


196. 

PT' 

grains,  about  .01  mm.  In  diameter, (Figure  97    ).  The 
etch-otruoture  of  the  individual  grains  usually  possesses 
one  set  of  strong  parallel  cracks,  or  less  commonly  two  sets 
of  different  intensity  and  persistanoe  roughly  at  right 
angles  which  is  characteristic;  of  orthorhombio  ohaloocite. 
The  size  of  grain  of  the  ohaloooite  indicated  in  this  way 
is  identical  with  the  grain  of  the  pure  bornite  front  the 
deeper  levels,  revealed  by  etching  and  tarnish,  and  was 
undoubtedly  inherited  from  it* 

Co.vsllite,  in  snail  amounts,  is  even  more  wide- 
ly distributed  than  the  chaloocite.   It  occurs  ae  a  sparse 
scattering  of  laths  in  a  one  of  the  most  massive  glance  of 
the  upper  levels,  and  thread-like  veinlets  of  it,  almost 
sub-microscopic  in  size,  break  the  bornite  in  the  deepest 
parts  of  the  ore.  81\i,nt  laths  of  oovellite  are  irregularly 
scattered  or  distributed  in  indefinite  bands  in  the  steely 
ohalcooite  from  the  ore -body  immediately  below  the  500  feet 
level.  The  covellite  is  slightly  attacked  by  the  chaloocite, 
but  the  amount  of  replacement  it  has  suffered  is  probably 
unimportant.  Where  the  relations  with  the  earlier  sulphides 
may  be  detected,  it  is  clearly  shown  that  the  oovellite  is 
almost  entirely  a  replacement  of  bornite.  Veinlets,  which 
are  merely  fine  threads  in  ohaloopyrite  or  tetrahedrite, 
widen  into  broad  bands  composed  of  rosette -like  sheaves  of 
small  oovellite  plates  when  they  puss  into  bornite.  In  a 
specimen  from  the  725  foot  level,  veinlets  of  bornite,  large- 


- 

l 

• 

iJLJ.       ,  C3J  J.  ^,  _,».». 


terfW 
:-ii*»J  10  e$ii  'd*   »all  xlw**  exe 

-^8 

'tioil  aacsio&qs 


137. 

ly  altered  to  oovsliite  but  still  containing  residues  of 
the  original  mineral,  cut  pyrite,  chalcopyrite  and  the 
gangue  minerals.   The  sharp  confinement  of  the  covallite  to 
the  limits  of  the  veinlets  clearly  indicates  the  greater 
resistance  of  the  other  minerals  to  the  attack  of  secondary 
solution. 

Tne  alteration  of  bornite  to  co.vellite  is  probably 
the  result  of  the  first  weak  effects  of  enriching  surface 
solutions,  and  many  of  the  covellite  grains  in  the  massive 
ohaioocite,  although  probably  not  all,  may  have  originally 
developed  in  the  bornite,  which  was  subsequently  replaced 
by  the  ohaloocite.  A  second  generation  of  covellite,  formed 
as  the  first  stages  of  the  oxidation  of  ohalcooite,  was  not 
observed  in  the  Magma  ores,  although  it  ia  a  common  feature 
in  several  other  deposits  studied. 

Small  spines  of  ohalcopyrite  occur  sparingly  in 
the  bornite,  associated  with  the  faint  blue  oovellite  vein- 
lets,  aud  are  probably  of  secondary  origin.  They  form  a 
very  small  fraction  of  the  total  amount  of  ohaloopyrite  in 
the  ore,  however. 


;.       '.! 


a   ?.. 


si 


$  at.il  »i 


;;sev 
to  as-o3i..3  . 


to 


1^ 


DIAGRAM  OF  MINERAL  SEQUENCE 


Period 


of 


ir.ineraliza  1  1  on 


Period,  of  oxiclatio; 


Quartz 

Sericite 

Pyrite 

Chaloopyrite 

Borriite 

Sphalerite 

Galena 

Tetrahedrite 

Coveilite 

Chalcooitd 

Cuprite,  etc. 

Karachi te 

Wulfenite 
Native  Copper 
Native  Silver 


Early 


It 


Late 


1 


9,' 
S-- 


199. 


Discussion  and  Summary 

Origin  of  the  Primary  .Ores.  The  close  association 
between  the  porphyry  and  the  ore  suggests  a  common  origin 
for  the  two.  The  dike,  however,  is  hardly  large  enough  to 
have  been  the  source  of  the  emmanations,  and  the  deviation 
of  the  metals  from  a  larger  and  deeper  body  of  igneous  ma- 
terial, from  whioh  ths  like  itself  may  be  regarded  as  an 
off -shoot  is  a  more  probable  hypothesis.  The  diabase  and 
dacite  are  clearly  ruled  out  of  the  field  of  possible  miner- 
alizing agents,  and  the  porphyry  is  the  only  rook  upon  which 
an  igneous  theory  of  origin  may  rest.  The  common  association 
between  the  ores  of  the  Southwest  and  porphyries  of  post- 
Carboniferous  and  pro-Tertiary  age  adds  weight  to  this  ex- 
planation of  the  origin  of  the  Magma  ore-bodies.  There  is, 
however,  in  this  deposit  no  direct  proof  from  the  character 
of  the  minerals  whether  the  thermal  solutions  were  direct 
magmatic  emmanations,  or  merely  heated  meteoric  waters  whioh 
had  obtained  their  metallic  burden  by  the  leaching  of  ore- 
bearing  rocks  below. 

The  ores  are  now  at  least  a  thousand  feet  below 
the  Tertiary  erosional  top  of  the  Carboniferous  limestone 
preserved  under  the  daoite,  and  this  may  be  regarded  as  the 
minimum  depth  at  whioh  they  could  have  been  formed.  In  the 
neighboring  Ray  district,  there  is  a  thousand  feet  or  so  of 
Cretaceous  volcanic  material  overlying  the  Carboniferous, 


. 

«.£       -ft 


• 


. 
-*-. 

rf-eio  £Xg.sK  ti^'j   ^o 

. 
•^w  oi       .  • 

wol  -jo  ie»l 

• 

• 
• 

^istfon 

, 


200. 

but  it  is  not  known  whether  it  extended  over  the  country  as 
far  west  as  Superior  or  whether  it  is  older  than  the  porphyry, 
The  ores  are  undoubtedly  later  than  the  post-Carboniferous 
porphyry,  and  earlier  than  the  Tertiary  daoite,  but  a  closer 
dating  does  not  seem  possible.  Probably  it  is  safe  to  assume 
that  the  ores  now  mined  were  formed  at  depths  not  greater 
than  three  or  four  thousand  feet. 

The  geologic  relations  and  the  character  of  the 
accompanying  rock  alteration  indicate  that  the  deposit 
should  be  classified  as  an  ore-body  formed  at  moderate 
depths  by  ascending  thermal  solutions. 

The  pyrite,  ohaloopyrite,  exoept  the  spines 
associated  with  covellite,  bornite,  galena,  sphalerite  and 
tetrahedrite  are  all  clearly  of  primary  or  hypogeae  origin* 
There  is  a  distinct  sequence  shown  by  their  relations,  which 
is  indicated  in  the  diagram,  but  it  is  believed  that  they 
were  all  formed  from  ascending  solutions.  The  pyrite  is 
clearly  broken  by  veins  of  both  ohalcopyrite  and  bornite 
(Fig.  95)  -ai  the  chalooi-yrite  is  for  the  most  part  some- 
what earlier  than  the  bornite. 

From  the  relations  exposed  at  the  time  of  his 
visit,  Ransome  expresses  the  opinion  that  the  bornite  is 
of  secondary  or  supergerie  origin,  although  he  frankly 
states  thut  the  evidence  is  inconclusive.  The  ore  was 
exposed  to  a  depth  of  800  feet  at  that  time,  an  1  there 
seemed  evidence  that  the  richest  bornite  ores  occurred  in 


• 


. 

J 

JeJUb 


(e-   . 

:1W 
,Tl 

t 

j      joirlj     ? 


201. 

two  ehoots  each  related  to  the  two  zones  of  contact  of 
the  dike  and  the  displaced  quartz! te  stratum.  This  re- 
lation Raneome  believed  to  be  most  easily  explained  c.s  a 
result  of  descending  surface  solutions,  which  had  derived 
their  copper  both  from  the  ledge  above  and  from  lenses  of 
copper-bearing  sulphides  near  the  base  of  the  Devonian 
limestone,  similar  to  those  which  occur  at  this  horizon  in 
the  neighboring  Lake  Superior  and  Arizona  Kine.  The  ding- 
ling  of  solutions  descending  along  the  dike  with  solutions 
migrating  along  the  quart zite  which  is  the  chief  water 
carrier  of  the  sedimentary  series  is  suggested  by  Ransome 
as  the  probable  oause  for  the  localization  of  the  bornite 
at  the  contact  of  the  porphyry  and  quartzite.  The  bornite 
is  regarded  by  him  as  a  replacement  of  ohaloopyrite,  on 
evidence  similar  to  that  presented  on  the  preceding  pages, 
and  it  is  recognized  that  the  chaloocite  is  largely  a  re- 
placement of  bornite.  These  relations  are  believed  by 
Ransoae  to  indicate  that  the  bornite  is  the  first  and 
deepest  product  of  secondary  enrichment  (supergene  altera- 
tion), and  is  an  intermediate  step  in  the  change  of  ohalco- 
pyrite  to  chalcooite. 

Later  developments  indicate  that  tha  bornite  con- 
tinues without  notable  change  in  relative  abundance  to  the 
deepest  levels  (1500  feet)  and  does  not  pass  into  lean 
pyrite-ohaloopyrite .  ores  with  de,  th  as  the  hypothesis  of 
a  secondary  origin  would  demand.  The  ore-shoots  parallel 


. 


.-• 


203. 


to  the  quartaite  horizons  proved  to  be  leas  definite  than 
the  first  exposures  had  indicated,  and  the  distribution  of 
the  bornite  in  general  shows  littls  relationship  to  the 
structure  of  the  sediments. 

At  Magma,  as  elsewhere,  bornite  is  found  to  be 
more  susceptible  than  the  ohaloopyrite  to  ohalcocite  en- 
richment which  is  shown  by  the  marked  manner  in  which  the 
ohaloocite  ie  confined  to  the  bornite  where  both  minerals 
are  present.   In  a  few  oases,  thin  margins  of  bornite  were 
observed  arouni  ohaloopyrite  residues  in  chalcooite,  and  it 
is  possible  that  this  small  amount  of  bornite  may  represent 
an  intermediate  product  in  the  enrichment  of  chaloopyrite  to 
ohalcocite.  But  even  here,  it  is  not  certainly  the  case,  for 
the  bornite  rias  themselves  may  be  residues,  perhaps  pro- 
tected a  little  longer  than  usual  by  the  oonoentration  of 
iron  derived  from  the  ohaloopyrite,  or  possibly  by  some 
galvanic  action. 

The  association  of  bornite  and  galena  in  structures 
whioh  can  be  interpreted  only. aa  contemporaneous  intergrowths 
or  as  replacements  of  bornite  by  galena  is  strong  evidence 
that  the  bornite  is  not  derived  from  descending  solutions, 
for  secondary  galena  is  unknown  in  copper  ores.  The  same 
argument  may  be  made  in  the  case  of  the  bornite  and  tetra- 
hedrite,  based  on  similar  relationships. 

In  summary,  the  persisteaoe  of  the  bornite  with 
depth,  its  relation  to  galena  and  tetrahedrite,  and  the  lack 


203. 

of  dependence  on  present  or  past  land  surfaces  or  on  the 
structure  of  the  sediments,  all  indicate  with  a  high  degree 
of  certainty  that  the  rich  bornite  ores  of  the  Magma  Mine 
are  of  primary  (hypogene )  origin.  The  replacement  Of  chal- 
copyrite  and  gangue  by  the  bornite  is  regarded  as  a  primary 
process,  similar  to  the  replacement  -of  pyrite  by  ohaloopyrite. 

The  Origin  of  the  Secondary  Ores.  The  ohaloocite 
may  be  regarded  without  question  as  a  product  of  descending 
solutions.  Microscopical  evidence,  supporting  the  field  re- 
lations, indicates  that  it  is  chiefly  a  replacement  of  bor>r 
nite.  No  graphic  structures  between  bornite  and  ohalcocite 

were  observed  in  the  ore.  In  some  oases  the  attack  of  the 

the 

ohaloocita  developed  a  pattern  not  unlike    "ice-cake  structure" 

1  A 

described  by  C.  F.  Tolman,  Jr.   In  one  specimen,  a  tarnish 
on  the  polished  surface  revealed  coarse  bornite  grains  appar- 
ently set  in  a  cement  of  finer  grains.  Where  ohaloooite  was 
developed,  the  finer  grained  matrix  was  attacked,  but  the 
larger  grains  apparently  remained  resistant.   In  advanced 
stages  of  ohalcocite  replacement,  they  appeared  as  islands 
surrounded  by  broad  streams  of  ohaloocite,  in  which  floated 
numerous  small  residues  of  the  finer  grains.  The  advance 
of  the  ohaloocite  is  probably  directed  merely  by  physical 
causes,  such  as  the  greater  surface  offered  by  the  finer 


1.  C.  F.  Tolman,  Jr,  Observations  on  certain  types  of 
chalcooite  and  their  characteristic  atoh  patterns, 
T.A.I.M.E.,  Vol.  54;  pp.  402-435,   1917. 


204. 


material,  and  not  by  any  variation  in  the  nature  of  the 
bornite. 

The  distribution  of  the  oaaloooite  agrees  with  the 
distribution  of  the  oxidation;  i.e.  it  seair.s  related  to  a 
surface  dipping  to  the  east  parallel  to  the  bedding,  as  has 
been  described.  This  may  be  attributed  to  two  causes,  both 
of  which  undoubtedly  were  of  importance,  (l)  the  influence 
of  the  different  strata  on  the  penetration  of  oxidizing 
solutions,  and  (2)  the  existence  of  a  land  surface  of  low 
relief  in  pre-daoite  times.  They  will  be  considered  in 
order* 

The  limestones  and  quartzite  are  both  pervious 
rocks,  and  rocks  in  which  it  is  commonly  observed  that 
surface  changes  easily  penetrate  to  considerable  depth. 
(Tintic,  Biabee,  Bingham. )  The  thick  diabase  sill  which 
forms  the  wall  rock  of  the  dike  below  the  sediments  ia 
rendered  dense  and  impervious  by  the  abundant  development 
of  serpentine  uralite,  a., a  offers  a  serious  check  to  descend- 
ing waters.  The  diabase  out-crops  in  a  band  a  couple  of 
hundred  feet  in  width  north  of  the  ledge,  and  dips  into  the 
ridge  at  an  angle  of  about  30*  parallel  to  the  overlying 
ae ii cents.  It  does  not  out-crop  on  the  southern  side  of  the 
ledge,  but  its  position  and  relations  are  indicated  clearly 
on  the  mine  sections.   (Figure*  ISA  and  15B.) .  The  greater  depth 
of  oxidation  to  the  east  may  be  attributed  to  the  increasing 
depth  of  the  sediments  in  that  direction,  and  to  the  barrier 


S05. 


offered  by  the  diabase  rising  to  the  west.  It  should  be 
kept  in  mind,  however,  that  the  slope  of  the  present  surface 
rises  to  the  east,  and  that  the  distance  of  the  lower  limits 
of  oxidation  below  the  surface  increase  more  rapidly  than 
indicated  by  the  quoted  depths  below  the  collar  of  the  shaft. 
From  the  physiographic  side,  there  are  two  surfaces 
of  importance  in  connection  with  the  uecondary  ores,  viz., 
the  pre-dacite  surface  and  the  present  surface.  Front  the 
strikingly  even  line  shown  by  tha  contact  between  the  dark 
lava  and  the  white  underlying  limestone,  it  is  clearly  seen 
that  thd  daoite  flows  were  poured  out  over  a  country  of  low 
relief,  in  this  neighborhood  at  least.  The  underlying  lime- 
stone beds  had  been  little  disturbed  in  pre-d&cite  time, 
and  conditions  of  slow  erosion  with  accompanying  deep  oxida- 
tion undoubtedly  prevailed.  This  period  was  brought  to  a 
close  by  the  floods  of  lava.  Superficial  changes  of  the  ore 
were  thus  checked,  until  the  faulting  and  subsequent  erosion 
which  broke  the  monotonous  plains  of  d&oite,  again  exposed 
them.  The  recent  surface  is  still  one  of  ateep  and  almost 
precipitous  slopes.  Mechanical  erosion  is  probably  the 
dominant  process  by  which  the  escarpment  is  retreating  at 
present,  and  its  rapidity  is  unfavorable  for  the  production 
of  deep  oxidation.  Of  the  two  surfaces,  the  older  seems 
more  competent  to  have  been  the  one  to  which  the  r resent 
deep  oxidation  is  related.  This  conclusion  is  further  supj- 
ported  by  the  rough  parallelism  between  the  bottom  of  the 


931 


al 


306. 

oxidized  zone,    and  the   liircstone-iaoite  contact. 

There   is  no  reason  to  doubt  that  part   of   the 
oxidized  and  secondary  ore  was  formed  in  relation  to  the  pre- 
daoite   surface,   and  on  the  whole,    it  is  very  probable   that 
the  greater  part  of  the  rich  chalcocite  bodies  were  foraed 
at   thtkt   time.     While   it  is  true   that  enrichment  can  keep 
ahead  of  a  rapidly  retreating  surface,   as  at  Binghan  canon, 
it  is  extremely  doubtful  if  the  massive  and  deep  ohaloooite 
in  these  large  ore-bodies,  which  represent  a  very  complete 
replacement  of  the  primary  sulphides,    could  have  been  formed 
so  rapidly.     From  the   structure  of  the  ridge  and  the  high 
country  to   the  east,    it  is  probable   that  tl.e  sill  of  diabase 
would  tend  to  act  as  a  dam,   retaining  water  above   its  level 
in  the  mountain  rather  than  allowing  it   to  migrate   down  its 
upper  surface.     As  there  is  abundant  water  in  the  mine  at 
present  the   sediments  were,   without  doubt,   well  saturated 
below  a  depth  of  500  feat  before  the  mine  was  opened.     Any 
flow  of  the  ground  water  under   the  present  structural  condi- 
tions would  tend  to  take  place  from  the  higher  country  east 
of  the  mine   toward  the   valley,   which  would  force   the  water 
up  the  beds  and  through  the  diabase  at  the  lowest  outlets. 
The  present  climate  is  extrerr.ely  arid,   and^er'e'  is -no  reason 
to  assume  a  depressed  water  level  due   to  less  humid  conditions 
in  the  past.     The  height  of  the  mine  above   the  valley  wash 
eliminates   the  possibility  that  the  present  higher  level 
of  the  ground  water   is  caused  by  the  accumulation  of  detritus. 


io 


307. 


From  the  various  linee  of  evidence  a  depth  of  about  500  feet 
below  the  shaft  u.ay  be  set  as  the  lower  limit  to  which  in- 
tunaive  oxidation  can  extend  from  the  present  surface,  and 
it  is  believed  that  the  deep  oxidized  ores  and  the  ohalood  te 
are  cost  probably  products  of  enrichment  related  to  the  pre- 
daoite  surface,  which  ie  now  tilted  to  the  east  at  an  angle 
of  about  30°  .  Studios  by  other  members  of  the  staff  of  the 
Secondary  Enrichment  Investigation  afford  impressive  evidence 
that  the  main  period  of  oxidation  and  enrichment  in  the 
neighboring  Globe  and  Miami  region  was  earlier  than  the  daoite 
and  the  subsequent  tilting,  which  greatly  strengthens  the 
similar  interpretation  in  the  case  of  the  Magma  deposit* 

Conclusion.  The  results  of  this  study  indicate 
that  the  rich  bornite  and  chaloopyrite  ores  of  the  Magma 

Mine  are  of  hydrothermal  origin,  and  formed  slightly  later 

i 

than  the  poat  -Carboniferous  porphyries  but  earlier  than 
the  Tertiary  lavas.  The  bornite  is  a  replacement  of  py- 
rite  and  chalcopyrite  in  part,  but  it  is  not  a  product  of 
descending  solutions,  except  to  an  almost  negligible  extent. 
The  rich  chaleocite  ore  -bodies  were  formed  largely  by  the 
replacement  of  the  primary  bornite,  and  are  a  product  of 
descending  surface  solutions  ohiefly  in  pre-dacite  time. 


OCCU.  JHI) 

OP 
fiQKfilZB 


Donald  Hamilton  McLetighlin 


?OL.   II 


KSNNSCOTT,  ALASKA. 
INTRODUCTION. 

The  ores  of  the  Kenneoott  district  are  pre-iom- 
•V 
inately  ehalcocite.   In  then,  bornite  attains  only  a  low 

A 

ran*  on  a  quantitative  basis,  but  it  assumes  a  position  of 
especial  interest  due  to  the  critical  evidence  it  presents  on 
certain  questions  of  genesis.   The  secondary  or  primary 
nature  of  the  chalcocite  ore-bodies  offers  an  important 
problem,  and  its  solution  depends  to  a  large  extent  upon  the 
meaning  of  the  bornite  residues  in  the  ore  and  upon  the  inter- 
pretation 3f  various  structures  of  the  chalcocite  and  bornite. 

In  addition  to  the  great  chalcocite  bodies,  which 
are  confined  to  a  limestone  formation,  bornite  ores  occur  in 
many  places  in  the  district  in  a  thicfc  pile  of  altered  basic 
lavas  Known  as  the  Nilcolai  greenstone.  They  are  similar  in 
many  respects  to  the  general  type  of  ores  associated  with 
basic  lavas  in  many  parts  of  the  world,  and  afford  an  in- 
structive contrast  to  the  chalcooite  ores.  Both  classes  of 
deposits  have  passed  through  a  similar  history  in  geologically 
recent  times,  which  provides  a  common  ground  upon  which  their 

• 

different  reactions  to  the  processes  of  surflcial  alteration 
may  be  compared. 

Situation.   Kennecott  is  situated  at  about  61°  30' 
north  latitude  and  14-3°  west  longitude  on  the  lower  spurs  of 
the  southern  slopes  of  the  great  Wrangell  Range  in  southwestern 
Alaska.  The  camp  is  at  an  altitude  of  :ibout  2000  ft.  on  the 


VI  i 


•  01  q* 


209 


eastern  side  of  the  broad  Kennecott  glacier,  which  descends 
from  the  ice-fields  of  Mt.  Blackburn,  (  161M-0  ft.  )  only  23 
miles  to  the  northwest.  The  Kenneeott  River,  which  springs 
from  the  end  of  the  glacier  five  miles  south  of  the  camp, 
drains  into  the  Nizina  River  which  soon  joins  the  Chitina, 
a  broad  stream  flowing  westward  sixty  miles  or  so  through  a 
wide  structural  depression  to  the  Copper  River,  whose  deep 
glaciated  v-illey  penetrating  the  Chugach  Range  affords  an 
outlet  to  the  Pacific  for  the  waters  of  the  entire  region. 
The  Copper  River  and  Northwestern  railroad  extends  from  Cor- 
dova on  Prince  William  Sound  to  Kennecott,  a  distance  of 

196  miles  by  way  of  the  carfona  of  the  Copper  River  and  the 

accessible 
Chitina  Valloy,  -\iA  makes  the  carap  at  all  seasons  of  the  year. 

A 

M ines.   The  Bonanza  and  the  Jumbo  yines,  v/hich  are 
worked  by  the  Kennecott  Copper  Corporation,  are  in  all  respects 
the  most  important  properties  in  the  region.   They  are  situated 
high  on  Bonanza  Ridge,  a  spur  extending  to  the  south  from 
the  lofty  Wrangell  Range.  Bonanza  Peak,  the  highest  part  of 
the  ridge,  rises  to  an  altitude  of  6950  ft.,  which  is  about 
1500  ft.  above  the  Kenneoott  glacier  to  the  v/est,  and  the 
narrow  valley  of  McCarthy  Creek  to  the  east.   The  mines  are 
less  than  600  ft.  from  the  crest,  and  in  an  extremely  rugged 
topography  due  to  the  intersecting  cirques  of  small  mountain 
glaciers.   The  mill  -and  other  works  are  situated  at  the  ter- 


9tti 


210 


minus  of  the  railroad  at  Kennecott  at  an  elevation  of 
ft.  The  ore  is  transported  to  the  mill  by  aerinl  tram-ways, 
which  are  also  used  for  carrying  supplies  and  members  of  the 
staff  up  to  the  mines.   The  Erie  Mine,  which  lies  about  four 
Mies  north  of  Kennecott  and  about  1000  ft.  above  the  glacier, 
hae  been  developed  to  a  slight  extent  by  the  oompiny,  but  as 
yet  littlo  ore  has  been  shipped. 

On  the  eastern  side  of  Bonanza  Ridge,  and  about  a 
half  mile  to  the  east  of  the  Bonanza  Mine,  is  the  Mother  Lode 
i.'ine,  the  property  of  a  smaller  company.  A  fair  amount  of 
development  work  had  been  done  in  1915,  an  aerial  tram- 
way built  to  McCarthy  Creefc,  and  some  ore  had  already  been 
shipped. 

All  of  the  deposits  at  the  mines  mentioned  thus  far 
are  in  the  Ohitistone  limestone,  and  the  ore  is  to  a  very 
large  extent,  chalooclte.   There  are,  however,  in  the  viein- 


ity  cf  Kennott  numerous  small  deposits  of  copper  minerals  in 

r 

the  Nikolai  greenstone,  but  little  work  has  been  done  on  any 
of  them.   To  the  east,  on  NiKolai  Creek,  a  deposit  of  this 
sort  has  been  slightly  developed,  with  results  somewhat 
promising.  Many  miles  to  the  west,  in  the  Kuskulan-a  and 
Kotslna  Districts,  several  similar  deposits  in  the  greenstone 
h«vo  bo?n  -vorked  in  a  small  way,  »nd  from  a  mine  on  Nugget 
Creak,  the  property  of  the  Alaska  Development  Co.  ,  a  little 
ore  hae  beer,  shipped.   Our  worx  at  Kennecott  did  not  allow 
time  to  visit  these  distant  deposits,  but  spsclnon*  and  des- 
criptions have  been  Kindly  furnished  by  Mr.  Alfred  ffandtke, 


•- 


211 


Literature.  The  region  has  been  mpped  and  described 
in  a  very  thorough  and  useful  manner  by  the  United  States 
Geological  Survey.   Their  most  recent  and  valuable  publica- 
tion for  locnl  information  is  Bulletin  '44ff,  The  Geology  and 
Mineral  Resources  of  the  Nizina  District,  Alaska,  by  F.  H. 
id  of  fit  an^  s.  R.  capps.  (1911).  The  accompanying  topographic 
and  geologic  maps  were  found  to  be  of  great  benefit  for  all 
questions  v/ithin  the  limits  of  accuracy  of  their  scale, 
(1/62500).  When  the  Survey  work  was  done,  however,  the 
nines  were  only  slightly  developed,  and  consequently  the 
published  descriptions  of  the  ore-deposits  were  of  little 
value  to  us.   A  list  of  earlier  papers  which  have  contributed 


to  the  Knowledge  of  the  region  is  given  in  Bulletin  WS  and  is 

1 
appended  below.    A  private  geologic  report  by  Professor  J.D. 

Irving  gave  valuable  Information  and  data  on  the  large  ore 

1.   Allen,  Lieut,  H.  T.,  Report  of  an  expedition  to  the 
Copper,  Tanana,  an-i  Koyukuk  rivers,  in  the  territory  of  Alaska, 
in  the  year  1SS5,  Washington  oov.  Printing  Office,  1#S7. 

Hi.yes,  C.  Willard,.  An  expedition  through  the  Yukon 
district:  Nat.  Oeog.  Mag.,  Vol.  '»-,  1*92,  pp.  119-162. 

hohn,  Oscar,  A  reconnaissance  of  the  Chitina  River 
and  Skolai  Mountains:  Twenty-first  Ann.  Report.,  U.S.O.S.,  pt.2, 
1900,  pp.  393-^0- 

Sohrader,  F.C.,  and  Spencer,  A.C.,  The  geology  -*nd 
mineral  resources  of  a  portion  of  the  Copper  River  district, 
Alaska:   Special  publication  of  the  U.S.O.S.,  1901. 

Mendenhall,  W.  C.  ,  and  Schrader,  F.  C.,  The  nineral 
resources  of  the  lit.  Wrangell  district,  Alaska;  Prof.  Paper, 
U.S.O.S.,  No.  15,  1903. 

Mentenhall,  W.  C.,  Geology  of  the  central  Copper  River 
region,  Alaska,   Prof.  Paper,  U.S.  G-.S.  ,  No.  4-1,  1905. 

Moffitt,  P.H.  and  Maldren,  A.G.,  The  mineral  resources 
of  the  Kotsina  and  Chitina  valleys,  Copper  River  region,  Bull., 
U.S.O.S.,  No.  ?iJ-5,  190^,  pp.  127-175. 

Keller,  H.  A.,  The  Copper  River  district,  Alaska,  Eng. 
?rv?  Win.  Jour.,  Vol.  *5,  No.  26,  June  190^,  pp.  1273-127^. 

Moffit,  F.  H.  ,  and  Maddern,  A.C.,  The  Kotsina-Chitlna 
region,  Alaska:  Bull.,  U.S.  a.  S.,  No.  37*f,  1909. 


al 


is 


bodies  mined  during  the  first  few  years  of  the  life  of  the 
Bonanza  Mine. 

The  "isometric  cleavage"  of  the  chalcocite  from  the 
Bonanza  Mine  was  first  described  by  Oraton  and  Murdoch,  and 
its  similarity  to  the  etructiire  of  synthetic  chalcocite, 
formed  at  a  temperature  of  1100°,  was  regarded  as  an  indication 
of  the  ^rimary  origin  of  the  mineral.   The  presence  of  bornite 
residues  in  the  chalcocite  and  the  possibility  that  it  was 

derived  from  the  bornite  by  replacement  was,  however,  inen- 

2 
tloned  later  by  L.  C.  Graton.   Analyses  and  specific  gravity 

determinations  of  chalcocite  from  the  Bonanza  Mine  made  in 

the  Geophysical  Laboratory  at  Washington  have  been  published 

3 
by  Posnjafc,  Allen  and  Merlin,  an-?  from  their  results,  it 

was  conclude^  that  the  mineral  was  formed  at  a  temperature 

above  91°.   Prom  the  results  of  a  microscopical  study  of  a 

§ 

suite  of  specimens  from  the  Bonanza  Mine,  0.  ?.  Tolrnan,  Jr., 

concluded  that  the  "isometric  structure"  of  the  Bonanza  ehal- 
eocite  vras  inherited  from  bornite,  and  that  the  greater  part 
of  the  chalcocite  of  the  deposit  was  a  replacement  of  bornite. 
A  statement  of  the  paragenesls  of  the  ore-minerals  is  made,  but, 
as  the  results  are  based  largely  on  laboratory  studies  without 
field  vorfc,  several  of  his  conclusions  may  be  criticised. 

1.  The  Sulphide  Ores  of  Copper,  T.A.I.k'.E.,  Vol.  M-5, 

p.  76*,  (  1913). 

2.  Discussion,   T.A.I.U.E.,  Vol.  M-ff,  1915-   P.  W- 

3.  The  Sulphides  of  Copper,  (  Secondary  Enrichment  In- 
vestigation, Contribution  No.  3),  2con.  Ueol.,  Vol.  10,  J»p.M-91- 

535. 

M-.  Observations  on  Certain  Types  of  Chalcocite  and  their 
Characteristic  Etch  Patterns,  T.A.I. U.S.,  Vol.  54- ,  1916;  pp. 
4-02-1+41. 


a\ii 


.- 


213 


Excellent  photographs  of  polished  surfaces  of  the  Bonanza 
ores,  accompany  the  article,  and  clearly  illustrate  certain 
structures  and  relations  of  the  ore-minerals. 

Ac-gnowledgeE'ients.   Our  work  was  greatly  assisted  by 
generous  information  and  many  courtesies  from  Mr.  H.  W.  Sea- 
grave,  the  manager  of  the  properties  at  Kennecott,  Mr.  H.  D. 
Smith,  superintendent  of  the  Bonanza  and  Jumbo  Mines,  and 
Mr.  W.  E.  Dunkle,  examining  engineer  and  geologist  for  the 
company.  Excellent  geologic  maps  of  the  mines,  prepared  by 
Messrs.  Smith  and  Bunkle  were  of  great  value  in  facilitating  on 
observations. 

Many  of  the  ideas  concerning  the  field  relations  of 
the  ores  that  are  presented  on  the  following  pages  I  owe  to 
L.  0.  Graton  anl  to  Dr.  A.  M.  Batenan,  but  the  intricate  way  Mi 
in  which  they  are  woven  into  the  entire  fabric  prevents  spe- 
cif io  recognition  in  the  text,  and  allows  me  i.ieroly  to  ex- 
press at  this  place  ny  appreciation  of  their  advice  and  val- 
uable criticism. 

GENERAL  GEOLOGY, 

Nikolai  greenstone.  The  base  of  the  visible  geologic 
column  near  Kerne  cott  and  in  t'ho  entire  district,  is  the 
Nikolai  greenstone.  The  formation,  as  exposed  on  the  slopes 
of     Bonanza  Ridge,  consists  of  a  thickness  of  a  little  over 
3000  ft.  of  altered  basaltic  flows.  The  base  is  not  recog- 
nized, SB  the  lower  boundary  is  due  to  faulting  for  the  most 


, 

- 

I 


part,  and  consequently  its  local  thickness  nay  be  consider- 
ably greater.   The  formation  Is  wide  spread,  extending  in  an 
irregulR.r  belt  beyond  the  Nigina  River  to  the  east,  and  with 
interruptions;  into  the  basin  of  the  Kotsina  River  to  the  west. 
From  several  good  exposures,  Moffit  and  Cappe  consider  it 
highly  probable  that  the  formation  has  a  maximum  thickness  of 
over  4-000  ft. 

:r.  viTvved  from  a  peak  near  the  nines,  the  formation 
is  seen  to  be  distinctly  stratified  in  rather  thick  beds 
probably  representing  individual  flows.  (  Figures  19»  21  ). 
Close  measurements  were  not  possible,  but  no  flows  were  recog- 
nized thinner  than  30  ft.   A  change  from  a  dense,  fine  grained 
rock  to  a  reddish  ainygdaloidal  type  was  observed  at  many 
horizons,  and  is  probably  the  expression  of  the  variation 
from  the  bottom  to  the  top  of  an  individual  flow.  The  dense 
rock  is  a  fairly  i-esh  cray  lava  with  a  fine-grained  diabasio 
texture;   the  amygdaloid \l  rocks  are  commonly  much  more 
altered,  ~nd  ire  of  a  dull  green  or  red  color.  All  gradations 
exist  oetreen  the  two  extremes.  Cliffs  are  frequent  -along  the 
horizons  of  the  denser  types,  while  the  reddish  rooks  seem  to 
\veather  preferably  to  benches., 

?ron  a  microscopic  study  of  many  specimens,  the  freeh 
rock  is  known  to  consist  of  laths  of  plagioclase,  (basic  ande- 

eine  for  the  most  part),  v;ith  grains  of  colorless  augite, 

is  accessory, 
Chirac teriRticilly  in  the  ophitic  texture,  i'agnetite^  and 

a  small  amount  of  chalcopyrite  an*!  rarely  other  copper  min- 
erals in  found.   The  rubordlnate  ground-mass  way  have  been 
glassy,  but  none  was  seen  in  in  unaltered  state.   The  alter- 


. 


it  ion  of  the  lavas  is  widespread,  and  even  in  the  freshest 
speeliaensjthere  is  aslight  development  of  secondary  materials. 
The  ground  mass  has  in  all  cases  ^one  to  radial  aggregates 
of  pennine  aiil  deleeaite.   In  most  specimens,  the  pyroxenes 
are  partially  or  wholly  altered.   Serpentine  Is  also  present, 
but  its  separation  from  chlorite  In  many  oases  is  very  dif- 
ficult. The  feldspar  is  uore  resistant,  but  in  the  most  In- 
tensely altered  types,  it  passes  into  an  aggregate  of  chlorite 
and  oilclte,  with  more  or  lees  kaolin. 

Zeolites  are  uncommon  near  Kennecott,  but  a  small 
amount  of  eplstllblte  was  observed  in  one  section,  probably 
as  a  replacenent  of  feldspar.  In  material  from  Elliott  Creek 
in  the  Kotslna  District,  heulandlte  occurs  In  a  olnilar  eo- 


lation, *na  datollte  is  abundant  in  snail  albite-bearlng  veins. 

1 
Soffit  reports  thoiasonite  in  a  specimen  from  Chltitu  Oreek 

2 
and  A.  Knopf  describes  zeolites  in  the  lavas  of  the  White 

River  country.  The  araygdule  fillings  are  largely  calclte, 
with  some  quartz,  and  ^hlorites.  Bornlte,  chalcopyrlte,  and 
pjrrlte  occur  In  the  aiaygdules  In  various  nlnorallzed  zones, 
and  in  a  few  oases  native  copper  was  observed.  The  relations 
of  these  minerals  will  be  dlBoussed  more  fully  under  the 
descriptions  of  the  mineralization  In  the  greenstone.  In  a 
few  places,  the  weathering  out  oi  the  amygdules  has  resulted  in 


1.   Bull.  U4g,   U.S.O.S.,  p.  61. 

Ph  .ivgdaloids  of  the  White  Plvor 

teflon,  Alnsxi,  Econ.  Oeol.,  Vol.  5,  PP.  2^7-256. 


Jae; 

at  si« 

.'P 


i    , 


216 


a  vesicular  root  with  a  striding  resemblance  to  a  recent  lava. 

The  greenstone  under  normal  conditions  of  erosion 
would  tend  to  yield  rather  smooth  forms,  but  under  the  severe 
attach  of  the  trioutary  glaciers  near  Kem.ecott,  the  upper 
slopes  of  the  ridges  are  precipitous,  although  the  cliffs  are 
uHually  not  as  bold  as  the  overlying  massive  beds  of  limestone. 
The  roofc  gives  rise  to  a  rich,  brownish-red  soil,  which  sup- 
ports a  luxuriant  growth  of  grasses  and  shrubs,  and  spruce 
on  the  lower  levels. 

The  age  of  the  greenstone  i:>  definitely  limited  in 
one  direction  by  the  overlying  Triassio  limestone,  but  the 
lower  limit  ia  uncertain.  On  account  or  the  similarity  between 

the  NiXolai  greenstone,  and  post-Carboniferous  volcanios  in 

1 
the  Skolai  Pass  country  to  the  northeast,  Moffit  and  Capps 

consider  it  probable  that  the  formation  is  of  early  Triassio 
age. 

Chit  is  tone^L  lines  tone.   The  Nikolai  greenstone  is 
overlain  by  a  sedimentary  formation  fcnown  as  the  Ohitistone 
limestone.   In  a  broad  way  the  contact  appears  conformable, 
the  v/ell  defined  bods  of  the  limestone  being  parallel  to  the 
somewhat  loss  distinct  flows  of  the  greenstone.  (  Figs.19,31*-  ). 
However,  v/here  mining  operations  have  exposed  the  relations 
ma- e  clearly,  and  in  a  few  other  cases,  there  is  evidence  of 
movement  along  or  not  far  above  the  contact.   The  movement 
was  probably  a  thrust,  but  in  general  it  is  not  of  sufficient 
importance  to  masfc  the  essentially  conformable  character  of 

1.   Loo.  cit.,  p.  63. 


fi 


217 


the  two  format ions. 

The  surface  of  the  old  volcanic  rocks  on  which  the 
ChitlBtone  formation  was  Inld  down  appears  to  have  been 
practically  flat,  perhaps  with  slight  Irregularities  here  and 
there,  but  with  no  notable  relief. 

The  first  deposit  consisted  of  about  three  feet  of 
mud,  now  consolidated  to  purplish  red  or  dull  greenish  shales. 
Near  Kenneeott  the  shale  is  a  persistent  bed,  and  encountered 
along  the  contact  for  many  miles.   It  tends  to  yield  a  bench, 
by  its  weathering,  which  forms  the  easiest  line  of  travel  in 
several  regions  of  difficult  cliffs.  (  See  Pig.3^o  ). 

The  shale  Is  succeeded  by  about  thirty  feet  of  an 
impure  silicioup  limestone,  made  up  to  a  notable  extent  of  flat- 
tened cylindrical  bodies  suggesting  fossils.   Numerous  frag- 
no  nts  of  crlnoid  sterna  were  found  in  this  horizon.  Above 
these  bods  occur  '1-0-50  feet  of  grey  limestone.   The  rock 
has  q  dull  appearance  on  the  froshly  fractured  surface  due 
to  its  very  fine  grain,  and  it  IB  oorriiaonly  known  to  the  miners 
is  ths  "dead  limestone".   The  chief  part  of  the  formation, 
which  overlies  this  type  of  rock,  is  a  crystalline  dolomltic 
limestone.  It  Is  separated  from  the  gray  limestone  by  a 
sharp  boundary  parallel  to  the  bedding  for  the  most  part  but 
riot  infrequently  brewing  across  ii;  irrsgular  v/ays.  Numerous 
inilyaes  made  by  Mr.  Dunkle  havs  shown  the  change  in  crystal- 
lino  character  of  the  limestone  to  be  closely  associated  with 
the  change  in  magnesia  content.   The  f  ine-gn  iao-1  limestone 


, 

is  fairly  pure,  yielding  9.30  /  Ca  CO^  with  3-9   Kg  CO,  , 
while  the  dolociitic  rock  averages  about66.7^Ca  003  and  30.  5 


The  ChitlBtonellrr.estone  is  estimated  to  have  a  thick- 
ness of  about  3000  ft.   It  passes  without  any  noticeable 
break  into  shales,  so  it  la  difficult  to  place  the  upper 
lii;iit  closely.   The  upper  beds  of  limestone  become  more 
and  more  inpure,  shale  partings  become  more  frequent,  until 
finally  the  true  line  character  of  the  foraation  i»  lost, 
and  the  rock  becomes  a  distinct  shale. 

The  McCarthy  Shale.   This  upper  formation,  which 
may  have  a  thickness  in  the  neighborhood  of  3000  ft.  ,  has 
been  termed  the  McCarthy  shale.  The  shale  is  entirely  re- 
moved by  erosion  from  Bonanza  Ridge,  but  it  occurs  to  the  east 
of  ±fe»  McCarthy  Creek,  where  it  is  exposed  to  good  advantage 
in  an  imposing  2000  ft.  cliff.   Fossil  evidence  has  shown  the 
limestones  anri  shale  to  be  of  Upper  Triaasio  age. 

Post-TriaBsic  Deformation.   After  the  deposition  of 
the  ./earthy  shale,  a  period  of  orogeuio  deformation  occurred. 
The  rocks  near  the  mines  were  folded  into  a  broad  anticline 
•-ith  a  NW-SJE  axis,  and  eroded  to  a  surface  of  gentle  relief. 
Tne  aro.sion  was  sufficient  to  expose  the  underlying  green- 
stone in  many  places,  for  the  overlying  sediments  of  the 
Jurassic  rest  on  all  three  of  the  foregoing  rock  divisions. 

Kenneoott  Formation.   The  McCarthy  shale  i«  suc- 
ceeded unconfonnably  by  the  Kenneoott  foraation,  a  series  of 
black  chalss,  .^ray  shales,  cherts,  sandstones  and  conglomerates., 


219 
which  from  its  best  exposures  has  been  estimated  to  be  at 

least  7000  ft.  thick.   It  is  known  to  be  of  Jurassic  age,  and 
rests  on  the  even  erosion  surface  cut  across  the  greenstone, 
and  the  Triassic  sediments. 

Hear  Kennecott  it  occurs  as  a  thin  veneer  of  shales 
and  cherts,  resting  on  the  greenstone,  and  preserved  only  in 
a  few  down-faulted  patches.  The  section  of  Bonanza  Ridge 
shown  in  Fig. IS  illustrates  the  probable  relations.   In  the 
canon  of  national  Creek,  running  up  from  the  Kennecott  Mill, 
a  thickness  of  several  hundred  feet  of  cherts  and  shales  is 


Southwest  Northeast 


Horizontal     anJ     verticali      sca/es    :          linc.li      =     /  mile 

J?lg.l#.  Section  through  the  Bonanza  Eidge  near  the 

Bonanza  Mine. 

exposed.  The  chert  occurs  in  distinct  beds  £-3  inches  thick, 
separated  by  partings  of  shale,  a  fraction  of  an  inch  thick. 
This  remarkable  alternation  of  cherts  and  shales  is  almost 

identical  with  that  found  in  the  Franciscan  series  in  Cal- 

1 
ifornia  (Jurassic,  radiolarian  cherts)  ,  and  again  in  the 

Monterey  series,  (Miocene,  diatomaceous  cherts)  (J?ig.  25  ). 
The  significance  of  the  rhythmical  change  has  not  been  de- 
duced as  yet.   The  gray  shales  are  exposed  along  the  lower 
course  of  Bonanza  Creek,  but  the  black  shales  of  the  formaticn 


1.  Hinde,  S.  J.t  Hote  on  the  radiolarian  chert  from  Angel 
Island  and  from  Buriburi  Ridge:  Calif.  Univ.  Dept.  Geol. 
Bull.,  Vol.  1,  Ho.  7,  pp.  235-E40;  1894.  Also  A.  C.  Lawson, 
The  SanPrancisco  Polio,  U.  S.  G.  S.f  (1915.) 


. 


; 


220 


were  observed  in  this  neighborhood  only  as  inclusions  in 
the  porphyry  of  Porphyry  Mt.   Conglomerate  boulders,  prob- 
ably of  the  Kenneeott  formation,  were  observed  in  the 
bed  of  National  Creek,  but  the  rock  in  place  was  not  found. 
With  the  exception  of  a  small  pooket  of  sandstone  probably 
of  Eocene  age  ?rest  of  the  glacier,  and  the  Quaternary  gravels 
of  the  valleyn,  the  Kennecott  formation  completes  the  aedi- 
mentary  column  near  the  nines. 

PorphyrloB.  The  Jurassic  rooks  are  cut  by  abundant 
intrusions  of  porphyritio  rocks,  chiefly  white  quartz  por- 
phyry.  The  largest  body  is  the  maes  which  forms  Porphyry  yt. , 
(  Pig. 22  )  in1'5  to  the  southeast  a  similar  body  is  exposed  on 
the  slopes  of  Sourdough  Kt.   The  quartz  porphyry  la  cut 
by  several  more  basic  dikea,  but  they  are  not  abundant,  and 
are  of  very  subordinate  volume. 

Under  thf;  microscope,  the  porphyry  is  seen  to  contain 
phenocrysts  of  quartz  and  plagioolase*  The  latter  is  com- 
monly acid  andesine,  but  it  is  as  acid  as  ollgoclase  in  some 
cases.   Orthoolase  phenocryats  are  rare.  Biotlte  aocurs  in 

iall  amounts,  as  short  laths,  usually  greatly  altered  to 
Muscovite;  hornblende  la  the  only  other  ferromagnesian  mineral, 
and  it  is  very  subordinate  and  usually  oxidized  to  linonlta 
at  the  surface.  The  ground  mass  is  micro-granitic,  and  is 
composed  commonly  of  an  interlocking  raino  of  small  quartz 
and  orthoclase  grains.  Nenr  the  edges  the  rock  has  a  glassy 


221 


ground  raise,  but  even  in  the  heart  or  the  area,  tho  grain  is 
axtreuely  fine.   The  rook  is  considered  a  quartz  dlorite 
porphyry  by  iiofrit  and  Capps,  from  a  wide  study  of  its  oc- 
currences. 

Tho  bodies  forming  Porphyry  rand  Sourdough  itts.  are 
referred  to  by  iiofflt  as  laccoliths,  but  in  no  place  was  I 
le  to  se  -,  any  evidence  that  they  had  made  way  for  them- 
selves by  thrusting  aside  the  older  rocks.  3oth  are  apparently 
intrusions  in  the  Kennecott  shales.   The  beds  along  the  edges 
or  th  porphyry  exposed  in  the  oanon  of  National  Creek  are 
intimately  intruded  by  sills,  but  are  not  noticeably  disturbed. 
The  rock  or  Porphyry  Mt.  contains  numerous  inclusions  of 
black  shale,  many  of  very  irregular  shapes,  which  stand  out 
in  striking  contrast  to  the  white  porphyry  host.  (?igs«  24,26) 

The  horizon  of  the  black  shale  in  this  vicinity  is  not  known, 

inclusions 
but  a  V0!-  iblc  interpretation  is  that  the  A  are  blocks  which 


have  settled  froir.  above  in  the  course  or  uagnatlc  stoping. 
The  contacts  or  the  porphyry  and  the  shale  zenoliths  are 
sharp,  i&d  there  is  no  evi     of  assimilation.  The  Irregular 
ron.iK  or  some  or  the  senoliths  must  be  attributed  to  mechanical 
causes  rather  than  chemical.  AS  the  porphyry  probably  made 
its  "'ay  upward  into  the  Jurassic  terrane.by  replacing  the 
beds,  arid  as  there  is  no  evidence  or  a  floor,  it  is  prefer- 

able to  refer  to  these  bodies  as  stocks  rather  than  lacoo- 

1 
lithe. 


1.   S'be  section  AA'  of  the  nap  accompanying  Bulletin 
r'o_*  the  ro?i;i  01'  the  Sourdough  i;t.  "laccolith." 


222. 


The  fine  grain  of  the  roci  is  surprising  in  such 
large  bodies.  Porphyry  Mt.  stock  is  about  five  miles  in  the 
north-south  diameter  <?nd  about  two  miles  in  an  east-west 
direction.   The  porphyry  mass  of  Sourdough  Mt. ,  '.••hieh  I 
saw  only  . Tor.  *  distance,  is  about  five  miles  in  diameter  in 
a  southeast-northwest  direction,  an'i  is  probably  continuous 
with  the  stoex  of  Porphyry  Mt.  under  the  gravels  of  McCarthy 
Creefc.     OM  the  distribution  and  dip  of  ths  Jurassic  sed- 

;ijts  near  the  western  base  of  Porphyry  Mt. ,  it  la  cloar 
that  the  cover  over  the  intrusion  could  not  have  been  thicx 
even  if  the  igneoue  rocx  did  not  penetrate  far  above  its 
present  positions.   The  fine  grain  is  probably  accounted  for 
by  thi"  condition. 

The  porphyry  intrueions  are  confined  to  the  Kenneoott 
formation  to  a  surprisingly  complete  degree.  Near  Keaiiecott, 
only  tv/o  porphyry  bodies  were  observed  in  the  older  roots; 
one  a  small  dike  cutting  the  Chitistone  limestone  at  the  Erie 
Mine,  ^n''  the  other  an  irregular  patch  of  porphyry  in  the 
greenstone  near  the  glacier  about  2  1/2  miles  above  the  Dill. 
In  all  cases  in  this  vicinity,  the  porphyry  occurs  :ts  intrusions 
in  the  Juraaric    B  -^nta.   Thin  relation  has  be  on  investigated 
with  eppacial  care  by  the  members  of  the  Survey,  and  11  has 
been  fouri-1  to  be  usual  throughout  the  entire  district.   The 
greater  ease  of  injection  of  the  n".graa  into  the  fissile  shales 
of  the  Acnnscott  formation,  than  i..i~  th<  BO     -;oive  older 
rocK.8  is  offered  by  Mofz'lt  and  Cappsas  an  explanation.  The 


223 


confinement  of  the  porphyries  to  these  areas  of  weli-bedded 
sediments,  may  mean  that  the  intrusions  were  liter  than  the 
ie format ion  of  the  Jurassic  rocXs.   The  places,  where  the 
Jurassic  sediments  now  occur,  are  mostly  basins  formed  by 
faulting  or  folding,  and  the  lower  position  of  the  fissile 
beds  1:;  these  areas  may  have  allowed  them  to  offer  the  easiest 
pathway  for  the  rising  magma.   If  the  porphyries  were  in- 
truded before  the  floor  beneath  the  Jurassic  beds  had  been 
deformed,  there  lr<  loss  reason  for  the  remarkable  absence  of 
the  intrusions  in  the  underlying  rocfcs  which  of  course  were 
penetrated.  No  upper  age  Unit  for  the  porphyries  can  be 
definitely  set.  They  are  not  earlier  than  the  late  Jurassic, 
and  are  probably  earlier  than  the  Tertiary  voloanlce.  They 
iaay  possibly  be  phases  of  the  great  igneous  activity  at  the 
close  of  the  Jurassic,  but  there  is  no  direct  evidence  to 
prove  or  disprove  this  view. 

The  Wrangell  Volcinics.   The  lofty  Wrangell  Range 
to  the  north  of  Kennecott  consists  of  a  great  series  of  lava 
i  lows  and  associate^  fragmented,  rocfcs,  which  range  in  age  from 
the  early  Tertiary  to  the  present  day.  ( Figs. 2S,30, 31  ). 
A  surauary  of  the  evidence  gathered  from  various  portions  of 
th5  range  indicates  that  the  volcanic  activity  probably comraencec 
in  the  Eocene,  but  that  the  greater  part  of  the  lavas  were 


1 

poured  out  Inter  in  the  Tertiary  and  In  the  Pleistocene. 

Andesitic  flows  from  Mt.  Wrangell  interbedded  with  glacial 

2 
riebris  are  described  by  Mofi'it  and  Maddren,  and  explosive 

eruptions,  of  tho  mountain  havo  occurred  within  the  list  few 
years.   The  volcanics  are  strikingly  v/ell  bedded,  and  rest  in 
nearly  horizontal  positions  on  a  surface  of  low  relief,  now 
at  in  altitude  between  6000  and  7000  ft.  As  seen  frora  Bonanza 
Peak  (Fig. 31  )  they  have  a  slight  dip  to  the  northwest. 
The  upper-portions  of  the  range  are  buried  in  snow  and  ice,  but 
there  can  oe  little  doubt  that  the  great  peats  are  of  volcanic 
origin.   Additions  to  Mt.  Blackburn  probably  ceased  a  fairly 
long  time  ago, for  the  mountain  is  strongly  eroded,  but 
1/t.  Wrangell  to  the  v/est  is  still  in  an  active  period  of 
growth,  and  appears  as  a  rather  flat  oone,  with  smooth,  little 
eroded  sides.v 

Structures.   The  conformable  greenstone  and  lime- 
stone formations,  which  are  the  chief  rocks  near  the  ore- 
bodies,  strike  northwest  and  dip  about  25°  in  the  vicinity 
of  the  Bonanza  Mine.  Fron  the  bold  exposures  on  the  cliffs, 
it  is  clearly  shown  that  the  dip  increases  alov/ly  to  the 
northwest,  and  on  the  western  side  of  the  Kennecott  glacier 
it  flattens  to  almost  zero  to  the  southwest.  (  figs.  28,29  ) 

1.   3ap.;rt,  s.  ii.,  The  Ghisana  -  white  River  District,  Alaska, 
Bull.  630,   U.S.a.S.,(  1516  ),  p.  61.  (Good  summary), 
ilendennali,   Loo.  cit.,  p.  57.- 

Sehrader  an-1  Spencer,   Lo<'.  cit.f  p.  52. 

*.   3ull.  371*-,  U.s.o.S., 


t 
I 


The  beds  apparently  fona  the  northern  limb  of  a 
broad  anticline  with  a  northwest-southeast  axis.  The 
structure  iu  Interrupted  on  the  south  by  the  porphyry 
intrucions  and  a  fault  which  has ; dropped  the  Jurassic 
beds  against  the  liiaestone. 

Near  the  Bonanza  Mine,  a  variation  in  the 
strlfce  N  15°  w  to  N  if5°  W  Indicates  the  existence 
of  a  small  cross  fold,  with  an  axis  pitching  with 
the  -lip  to  the  northeast. 

There  is  clear  evidence  in  the  mine 
wordings  and  elsewhere  of  movement  along  a  surface 
roughly  parallel  to  the  bedding  about  50-60  ft.  above 
the  greenstone-limestone  contact.  The  magnitude  of 
the  slip  along  this  surface,  which  is  commonly  referred 
to  as  the  "flat  fault"  is  not  Xnown.  In  places  it 
is  accompanied  by  a  strong  zone  of  breoolated  rook 
and  p;ouge.  On  the  olirfs  of  the  sharp  peax  which  rises 
between  the  Konnecott  Glacier  and  its  tributary  from 
i£t.  Regal,  there  is  evidence  of  thrust  faulting  parallel 
to  the  contact.  (Fig.  28  ).  The  limestone  bods  are 


locally  overturned,  and  a  broad  wedge  of  the  limestone 
saems  driven  Into  the  underlying  greenstone.  The  struc- 
ture la  eomplioatsd,  but  from  a  atudy  with  the  field 
glasses,  we  felt  that  It  afforded  good  evidence  of 
compression  In  the  llraeetone  parallel  to  the  greenstone 
nontaet.  From  this  condition  vrest  of  the  glacier,  it 

is  probable  that  the  "flat  fault"  at  the  mines  is  a 

there  is 
reverse  fault,  although  no  evidence  of  any  folding  of  the 

A 

beds  on  the  eastern  side  of  the  glacier. 

The  limestone-greenstone  contact  is  a 
strikingly  bold  feature,  due  to  the  sharp  contrast 
in  color  of  the  two  rooks,  (see  Figs. 19, 20, 29,3^  ), 
and  its  course  may  be  followed  for  many  miles  along  the 
bare  upper  slopes  and  steep  cliffs.  The  line  is  re- 
markably regular,  but  in  a  few  places  it  is  broken  by 
oroos-faults.  South  of  the  Bonanza-  Mine,  a  fault  has 
dropped  the  block  on  the  southern  side  about  300  ft. 
(Fig.  20  ),  and  on  a  spur  south  of  the  Jumbo  Mine,  there 
is  a  similar  fault  with  about  the  same  throw.  All 
other  breaks  observed  were  of  small  displacement. 


- 

• 

•     . 

• 

• 

'• 

/• 

. 

,i*il  Xt " 

•    •     t     , 

. 

i' 


PHYSIOGRAPHY 

The  land  surfaces  which  may  be  recognized  as  important 
stages  in  the  geological  history  of  the  district  are  fire  in 
number.   They  are  as  follows: 

(1)  The  surface  of  the  greenstones  on  which  the  Chiti- 
stone  formation  was  deposited; 

(2)  The  surface  out  across  the  disturbed  greenstone  and 
Triassic  beds,  upon  which  the  Jurassic  sediments  were  laid 
down; 

(3)  The  Tertiary  surface  on  which  the  Wrangell  volcanics 
were  poured  out ; 

(4)  The  pre-glaclal  surface;  and 

(5)  The  present  surface. 

Pre-Chitistone  Surf ace*  -  The  great  pile  of  laras  repre- 
sented by  the  greenstones  was  little  disturbed  or  eroded  be- 
fore the  overlying  shales  and  limestone  of  the  Chitistone  form- 
ation were  deposited.   It  has  been  urged  that  the  evenly  dis- 
tributed amygdules  and  the  concordance  of  the  flows  and  the 
sediments  indicate  a  submarine  origin  for  the  volcanic  rooks. 
Prom  the  sections  along  which  I  studied  the  greenstones,  how- 
ever, I  found  a  distinct  confinement  of  the  amygdules  to  def- 
inite horizons,  which  were  with  little  doubt  the  surfaces  of 
the  individual  flows.   Limonitic  alteration  is  far  more  pro- 


1  -  J.  D.  Irving,   "rivate  report  to  the  Kennecott  Copper 
Corporation. 


i 


. 


22% 


nounoed  in  such  portions  of  the  rock,  and  ia  strong  evidence 
of  a.  subaerial  origin.  According  to  this  interpretation,  the 
upper  surface  of  the  greenstones  was  once  a  land  area. 

Pre-gonnooott  Surface.  -  The  surface  upon  which  the  Juras- 
sic sediments  were  deposited  is  the  first  of  importance  in  pos- 
sible relation  to  the  secondary  alteration  of  the  great  oro- 
bodies  in  the  limestone.  Hear  Kennecott  there  are  few  oppor- 
tunities to  study  the  character  of  this  surface  in  detail,  but 
as  the  Jurassic  shales  and  cherts  rest  on  the  greenstone  (Fig.lS) 
it  is  evident  that  a  period  of  profound  erosion  preceded  their 
deposition.  Elsewhere  in  the  general  region,  the  lower  beds 
of  the  Konueoott  formation  have  been  seen  resting  on  the  evenly 
truncated  edges  of  both  the  greenstone  and  Triassio  formations. 
Erosion  of  the  pre-Jurassic  structures  had  clearly  advanced  well 
toward  peneplai nation  before  the  Jurassic  sediments  were  de- 
posited. 

Pre-Volcanic  surface.  -  The  next  land  area  of  which  there 
is  a  definite  record  ia  that  preserve!  beneath  the  Tertiary  vol- 
canic s  which  form  the  higher  altitudes  of  the  V/rangell  mountains. 
Seen  from  Bonanza  Peak,  the  contact  of  the  flat  lying  lavas  and 
the  earlier  formations  presents  a  strikingly  oven  line. 
(Fi,T3.30  and  31  ).  The  surface  truncates  the  older  beds,  and 


1  -   JJDffitt.  ?.  H.  and  Capps,  S.  R.,  Bull. 448,  U.3.O.H., 
Plate  VII,  opp.  page  36. 


jtWQB. 


>OCf 


'BO 


229 


represents  a  land  area  of  very  slight  relief.  Toward  the 
western  end  of  the  Wrangell  group,  however,  Mendenhall  de- 

scribes irregularities  as  great  as  3,000  feet  vertically  in 

1  2 
this  pre-voleanio  surface,   fcut  for  the  moat  part,  deserip- 

3 
tions  and  photographs  from  neighboring  districts  show  it  to 

be  similar  to  the  conditions  observed  north  of  Kenneoott. 
The  pre-volcanic  surface  most  probably  extended  over  the  Zen- 
nee  ott  region,  about  a  thousand  feet  at  least  above  the  high- 
est peaks  east  of  the  mines,  if  its  present  low  dip  may  be 
projected.   The  lavas  extend  south  on  Bonanza  Hidge  to  a 
point  within  six  miles  of  the  mines,  but  it  is  not  known 
whether  they  ever  continued  much  farther  south.   Areas  of 
flat  or  gently  rolling  country  at  altitudes  between  5,500  and 
6,500  feet  were  observed  on  ridges  west  of  the  Kenneoott  gla- 
cier (Fig.  29  )?  south  of  the  Chitina  Valley,  east  of  the  Hiaina 
River  and  elsewhere,  and  they  may  represent  remnants  of  this 
pre-volcanic  early-tertiary  surface,  which  was  not  buried  by 
the  lavas.   It  is  possible,  however,  that  these  surfaces  may 

1  -  Loc.  Git.,  pp  56  -  57. 

£  -  Shrader,  if.  C.  and  Spencer,  A.C.f  The  Gool.  and  Min. 
Resources  of  a  portion  of  the  Copper  Rivor  dist,  Alaska, 
U.S.G.S.,  Special  pub.,pp  51-52,  1901. 


3  -  Capps,  S.R.,  Bull.  630,  U.S.G.S.,  Che  Chiaana  -\vhite 
River  Dist,  Alaska,  Plate  IV,  p.  18,  Pig.4,  p.  38. 


• 

. 


have  been  developed  later  as  an  early  stage  in  the  pre- 
glacial  stream  erosion,  of  the  region. 

Pre-Glaoial  Surface*  -  The  main  drainage  channels  such 
as  the  Chitina  River,  are  along  prominent  structural  lines, 
as  pointed  out  by  Moffit  and  Capps,  and  are  believed  by  them 
to  have  been  established  at  a  fairly  early  date*  The  earliest 
dissection  of  the  7/rangell  Range  was  probably  by  stream  ero- 
sion, and  a  rugged  topography  is  believed  to  have  been  in  ex- 
istence before  the  intense  glaciation  of  the  Pleistocene*  The 
remnants  of  flat  surfaces  mentioned  in  the  preceding  paragraph 

may  be  correlated  with  evidence  from  neighboring  parts  of  t  he 

2 

range,  and  their  altitude  and  distribution  make  it  very  prob- 
able that  ihey  were  developed  in  the  early  stages  of  the  ero- 
sion of  the  region.   For  the  most  part  they  are  too  low  to 
agree  with  the  pre-volcanio  surface.   Rejuvenation  of  the 
streams,  due  to  a  broad  uplift  of  tho  region,  may  be  con- 
sidered the  cause  of  the  dissection  of  these  surfaces,  which 
are  now  preserved  only  on  the  lower  ridges  that  flank  the  main 
range.  That  mountain-making  movements  continued  until  very  re- 
cent times  in  the  Wrangell  Bange  is  shown  by  the  disturbance  of 

1  -   Bull.  448,   pp.  74-75. 

2  -   Capps,  S.R.,  Bull.  630,  Plate  XVII,  opp.  p. 76. 


- 

.  TifC    £r  I 

Sejieiltfeteo  aeetf  0«*&. 
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231 


the  lava  beds  and  even  of  early  glacial  deposits  in  the 
Chisana  -  White  River  District,  described  by  Capps.  Con- 
sequently uplift  in  the  Kennecott  region  at  a  fairly  late 
time  is  not  out  of  accord  with  the  evidence  elsewhere. 

It  is  difficult  to  estimate  the  degree  to  which  the 
country  was  dissected  by  stream  erosion  before  the  advance  of 
the  glaciers.  The  Chitina  Valley  was  probably  similar  in  its 
broad  features  to  the  present  topography,  and  Moffit  and  Cappa 
believe  that  the  tributary  stream  drainage  was  well  developed. 
We  could  find  no  evidence  opposed  to  this  view.  It  seems  Tory 
probable  that  in  the  long  period  in  the  late  Tertiary, during 
which  the  volcanics  and  their  elevated  platform  were  exposed 
to  erosion,  a  topography  at  least  approaching  maturity  had 
been  developed. 

glaoirition  and  the  Present  Surface.  -  The  present  land 
forms  have  been  profoundly  modified  by  the  intense  glaciation 
of  the  period  which  is  probably  now  drawing  to  a  close.  In 
the  high  altitudes  glaoiation  bus  undoubtedly  been  long  con- 
tinued, but  as  outlined  above, stream  erosion  is  considered  the 
chief  agent  rtiich  h^d  shaped  the  earlier  drainage  lines.  Upon 
these  channels  the  great  glaciers  of  the  Pleistocene  have  im- 
pressed their  characteristic  features,  eroding  broad  U-shaped 
valleys  in  place  of  the  narrower  canons,  truncating  projecting 
spurs  and  straightening  the  valley  walls,  and  over-riding  and 


7    3:  .  t"X^.'o. 


smoothing  the  lower  ridges.   Irom  the  difference  in  elevation 
between  the  mouths  of  hanging  tributary  valleys  and  the  main 
glacial  channels,  Moffit  and  Capps  estimate  the  amount  of  deepen 
ing  by  glacial  scour  to  have  been  between  1,000  and  1,500  feet. 

\ 

So  evidence  of  more  than  one  period  of  glaciation  has  been 

2 

observed  in  the  Kennecott  district.  S.  R»  Capps,  however,  has 

recently  described  indurated  and  deformed  glacial  depobits 
near  the  source  of  the  VJhito  Kiver.   They  are  associated  with 
lava  flows,  and  are  overlain  unconformably  by  recent  glacial 
deposits.   The  evidence  is  interpreted,  however,  as  the  record 
of  an  early  Pleistocene  glacial  advance,  separated  by  a  long 
time  interval  from  the  latest  glaoiation,-  but  nevertheless  part 
of  the  same  large  period.   from  this  information  it  seems  fair- 
ly probable  that  there  hus  been  similar  early  glaciation  through 
out  the  Wrangell  mountains,  but  in  moat  places,  as  near  Konne- 
cott,  the  intense  erosion  of  the  recent  glaciers  has  obliterated 
all  earlier  records. 

The  glacial  period  in  thia  region  has  not  yet  passed. 
The  broad  valley  to  the  west  of  the  mines  is  still  occupied  by 
the  great  Kennecott  Glacier,  (i?igs.  28  and  29  )  which  descends 
from  the  ice-fields  of  Mt.  Blackburn  (16,140  ft.)  to  an  alti- 

1  -   Bull.  440,  p.  44. 

2  -   Two  glacial  stages  in  Alaska:  Jour. "eol.,  Vol. ii3, 
pp.  748-756,  1915,  and  Bull. 630, U. S. (J. S  .,  pp. 63-67,  1916. 


3r    »c  tot   £ 

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233 


tude  of  only  1,400  ft.   Bonanza  Peak  still  supports  five  abort 
mountain  glaciers,  and  even  now  one  side  of  the  outcrop  of  the 
Bonanza  ore-body  is  attacked  by  a  small  glacier,  which  en- 
riched its  moraine  with  dhaloocite  until  mining  operations 
interrupted  it.   lilsewhere  along  tho  ridge,  especially  on 
Porphyry  Mt.,  the  cirques  are  occupied  by  "rock  glaciers", 
(Fig.  22  )  which  may  be  regarded  as  the  dying  efforts  of  true 
glacial  action. 

.in  attempt  is  made  by  Moffit  and  Capps  to  outline,  on 
their  geologic  map,  the  land  areas  which  projected  above  the 
glaciers  at  the  time  of  theiriaaxinun  development.  The  line 
follows  the  topographic  breax  between  the  smoothed  and  rounded 
contours  of  the  lower  slopes  and  the  rough  and  angular  forme  of 
the  summit  regions.   In  general  it  may  be  drawn  with  reasonable 
certainty,  but  near  the  mines  above  Xennecott,  the  sharp  arretes 
and  difficult  peaks  are  clearly  due  to  the  intersecting  cirques 
of  the  present  glaciers.   .01  earlier  topography,  smoothed  by 
over-riding  masses  of  ice,  could  easily  have  been  destroyed  by 
these  later  changes;  consequently  the  character  of  the  present 
summit  topography  cannot  be  regarded  as  conclusive  evidence  that 
tho  entire  ridge  had  not  been  ovor- ridden  by  the  ice. 

There  is  a  slight  amount  of  post-glacial  erosion  along  the 


1  •    Cappa,   .  ..,  Rook  glaciers  in  Alaska:  Jour. (Jeol  ..Vol.  18, 
pp.  359-375J   Also  Bull. 448, pp. 52-59. 


;n  .; 

WO 

Dl 


333A. 


VII 
Kennecott.   llq.aka. 


Fig.   19.     The  greenstone   -     limestone  contact  along  the 
oliffa  north  of  the  Bonanza  Mins. 


Fig.  20.     Fault  breaking  the  greenstone-limestone  contact 
southeast  of  the  Bonanza  Mine. 


333B. 


PLATE  VIII 

Ker.neoott.   AlaaV    . 


Fig.   21.     The  spur  along  which  the  veins  cf   the  Bonanza 

Mine  out-crop.      Note  mine  workings  on  the  cliff 
faces. 


Fig.   22.     View  to  the  south  from   the  surcmit  of  Bonanza 
Peak.     Porphyry  Mountain  in  the  centre,   the 
Nizina  Riul   Chitina  Valllas   in  the  back  ground 
an3   the  Chugach  Mountains  on  the  horizon; 
McCarthy  Creek  caflon  to  the   left;   a  little  of 
the  moraine  of  the  Kennecott  glacier  on  the 
extreir.e  ri  sht. 


. 

"^'  '  * 
'  *  •  -  J 


•a»i 


333C, 


PLATE  IX 
Kennecott.  Alaska. 


Fig.  27.  The  Chitistone  limestone  in  Independence  Basin, 
southeast  of  Bonanza  Mine.  The  spur  on  which 
the  Bonanza  Mine  is  situated  is  shown  in  the 
background  on  the  extreme  left. 


Fig.  "'-.  An  inclusion  of  black  shale  of  the  Kennecott 
formation  in  porphyry  on  northern  aide  of 
Porphry  Mountain. 


Fig.  25.  Chert  and  shale  beda  of  the  Kenneoott  formation 
;ng  National  Creek  above  Kennecott. 


233D. 


PLAT?  X 
Kenneoott,   Alaska... 


Fig.  26,     An  inclusion  of  ahale  oi    the  Kenneoott  formation 
in  porphyry  on  the  north  side  of  Porphyry  Moun- 
tain. 


Fiz.   ~7.      Inclusion  of   shale  in  porphyry,  cut  by  a  dike  of 
a  more  bassic  type. 


233E. 


PLATE  XI 
Kenneoott,  Alaska* 


Fi».  2f.     The  Kennecott  glacier   an -I  Mt .  Blackburn  from  the 
summit  of  Porphyry  Mountain.     Note   the  p;reenatOTie- 
limestone  contact  on  both  aides  of  the   glacier , 
The  fold  and  reverse-fault  above  the  contact  ar^ 
shown  on  the   cliff  between  the  main  glacier  and 
the  tributary  on  the  right. 


Fig.  29.     The  Kennecott  glacier  seen  frorr.  the  summit  of 
Bonanza  Peak.     Note   the   greenstone-limestone 
contact  along  the  cllffa  on  both  aides  of  the 
glacier,   rn'    the  flat  surface  on  the  moun- 
tains on  the  west  side.     The  peaks   to  the 
right  are  of  volcanic  material.     The   tribu- 
tary glacier  from  Mt.  Hegal   joins  the  Kenne- 
cott glacier  in  the   foreground. 


233F. 


PLATE  XII 


Fig.  ?0.     View  of  the   Wrangell  Range   looking  noi'th  from 
the  9twirr.it  of  Bonanza  Peak.     Mt.  Regal  and  the 
great  ice-fall  of   the  southern  tributary  of   the 
Kennecott  Glacier  are  on   the  right  of  center . 
Note   the  horizontal  be is  of  voloanio  rocka  which 
form  the  higher  peaks. 


View  of  the  valley  of  McCarthy  Creek  from  the 
summit  of  Bonanza   Peak,   looking  to  the  north- 
east.    Chltiatone   lirreatone  beia  form   the  cliffs 
in  the  foreground,  wi  '.h  softer  beds  of  McCarthy 
shale  above,   *»nd  the  sediments   (chiefly  shales) 
of   the  Kennecott  formation  on  the  smoother  dis- 
tant slopea.     The  volcanic  bedsi  of  the  higher 
peaks  are  prominsnt.     This  view  is  pBnoramic 
with  Fig.  70. 


233G. 


PLATE  XIII 
Kenneoott,   Alaaka, 


Fig.  3".     The  Jumbo  Mine  ani  Glacier,   sn3   the  northwestern 
cliffs  of  Bonanza  Pet-    . 


Fig.  "5^.     The  Jumbo  Mine   ->n;   dlscier,  beneath  the  southern 
cliff  of  Jurcbo  Caatle. 


Fig.  Ik.     The  limestone-greenstone  contact  along  the  cliffs 
north  of  the  Jumbo  Mine.     The  outcrop  of  the 
Jurrbc   Vein  is  njarke.1  by   the   airall   adit   on  the 
nearest  spur  on  the  right  side  of  the  picture, 
Just  above  the  oontaot.     The  Tram     House  of  the 
Jxurbo  Mine   is   shown  in  the   lower  ria;ht  fore- 
ground.    The   Kannecott  Glacier  ir>ay  be   seen   to 
the   left.      The  view  ia   panoramic  with   Fi».  33. 


£33H. 


PUTS  XIII   A 
Korneoott »  Alaska... 


FIT.  3i(.a .     Berp;sohrund  of  the  Jumbo  Glaoier,     Northwest 
aide  of  Bonanza  Peak. 


.   3J* .     The  contact  of  the  Chltistone   limestone  and 
Nikolai   greenstone    (beneath)   a-c  a   prospect 
near  the  Erie  Mine.     The   thin  shale  bed  at 
the  bottom  of  the   limestone  is  well  shown. 


Fitt.  "^-'1.0.     The  bench  alone;  the   liraestone-^rsenstons  con- 
tact nep.r  ths  f^rie  about   1000  ft.  a- 
bove  the  Kennecott  Glacier. 


lower  courses  of  the  streams  from  Bonanza  Ridge,  Tout  as  the 
glacial  period  is  still  in  existence  near  the  mines,  water  is 
an  unimportant  agent. 

MINERALIZATION  IN  THE  OREENSTONE. 

The  mineralization  in  the  greenstone  is  a  formation 
•vide  feature.  Traces  of  Copper  are  coupon  in  nearly  all  parta 
of  the  region  where  the  ronxo  are  exposed,  but  few  prospects 
give  ouch  promise  of  eventually  supporting  mine'-. 

?oria  of  Deposits.  The  mineral  deposit:?  in  the 
greonston'B  oo<->ur  in  tho  following  I'oras  (1)  veins  brea&ing 
morose  the  bedding  ox'  the  flows,  (2)  disseminated  deposits 
of  replacement  origin,  and  (  3 )  anygdule  fillings.  The  first 
group  is  the  most  important  from  a  commercial  standpoint. 
The  veins  in  the  Kennecott  district  ire  rather  short,  and 
few  were  observed  of  sufficient  perils tones  to  carry  them 
through  tho  thicJcnoss  of  more  than  two  or  three  of  the  flows. 
For  the  niopt  part  thoy  .  are  small,  and  are  to  be  measured  in 
inches  or  fractions  of  an  inch,  but  in  a  few  exceptions  they 
attain  a  thicfcnosa  of  several  feet,  with  their  other  diuensione 
correspondingly  greater. 

The  disseminated  ninerilizatlon  is  either  related 
to  definite  veins,  or  to  shear  zones  along  which  circulation 

01  solutions  could  tajce  place.   No  ores  of  this  sort  are  Known 

large 
to  bs  of  value  although  ar-;iK  o:'  the  .-rr-jsnstons  contain  suail 

^ 

A 


235 


iiuountn  of  copper. 

The  distribution  of  the  aniygdaloldal  deposits  is 
of  course  limited  to  horizons  of  once  vesicular  lava,  and  llite 
the  disseminated  mineralization,  there  is  a  dependence  on 
neighboring  veins  or  sheared  zones.   Near  Konueoott,  deposits 
of  this  sort  are  not  common,  although  native  copper  and 
bornite  v/ere  observed  in  amygdulss  in  a  few  cases. 

Gangue  Minerals.  Calcite  and  quartz  are  the  com- 
monest r>;angue  minerals  in  the  veins,  although  in  some  cases 
epidote  becomes  of  equal  importance.   The  veins  are  of  re- 
vlicenent  origin  for  the  most  part,  as  ia  clearly  shown  by 
the  relations  of  their  minerals  to  the  wall  work,  but  in 
seven!  localities  lar.^e  quartz  crystals  lining  cavities 
v-ore  observed,  and  there  could  be  little  doubt  that  open 
spaces  had  existed,  and  probably  played  an  important  role. 
Snail  veinlots  of  albite,  usually  associated  'vith  epldote, 
•ire  found  fairly  coononly,  and  in  a  few  cases, datolite  ^as 
observed  associated  with  the  other  ganguo  ninerals. 

The  araygdule  fillings  are  very  probably  of  the 
eaLie  origin  as  the  vein  fillings.  Pormiivs,  delessite,(  and 
possibly  other  chlorltes),  serpentine,  and  calcite  are  the 
usual  minerals.  ?ine  green  borders  of  fibrous  serpentine  were 
apparently  the  first  to  fora;   a  broader  layer  or  plaooroio 
"hlor:       uine  probably)  corcaonly  succeeds  the  serpentine, 
passing  toward  the  center,  and  the  interior  of  the  cavities 
in  many  cases  i:  formed     -oader  flafces  of  delessite. 


SJJ 


Calcite  is  the  iaet  to  form,  and  where  it  occurs,  it  is  nearly 
always  in  the  center.   Quartz  and  epidote  are  lese  common 
In  the  araygdulea.  A  mineral  resembling  prehnite  in  most 
or  its  properties,  but  not  positively  identified,  oooure  with 
native  copper  in  a  few  oases,  but  the  araygdules  composed  of 

zeolites,  and  native  copper,  which  are  plentiful  in  the 

1 
White  River  district  in  similar  rocks,  were  not  observed  in 

the  Kenneoott  region. 

ROCK.  Alteration.   The  rook  alteration  accompanying 
tho  miner a 112 at ion  ia  similar  to  the  general  propyilitization 
of  the  greenstone,  but  more  intensive.   Chlorite  is  more 
abundantly  developed  near  the  veins,  and  little  of  the  original 
Character  of  the  work,  generally  remains.  Heulaniito  and  a 
few  other  zeolites  less  positively  identified  (  probably 
harmotone  and  eplstilbite  in  two  oases)  replace  the  feldspar 
near  certain  veins.   Caloite  is  wide-spread  as  the  final 
product,  replacing  nearly  all  pro-existing  minerals. 

The  order  of  Liinorai  succession  can  not  be  stated 
in  detail,  but  for  the  veins  and  the  replacements  of  the  wall 
rock,  albite,  epidote,  quartz  find  datolite  may  be  regarded 
as  representatives  of  the  earlier  phases  of  the  mineraliza- 
tion.  Similarly  the  zeolites  and  calcite  belong  to  the 

closing  phases.  .>,. 

Plr' 
Primary  Ore-Miner  a  Is.       Bornlte  and  chaloo**te  are 

the  most   important  of  the  ore-minerals.      In  some  cases,   pyrlte 
1.      Knopf,    A.,    Loc.    ••••it. 


ir.-  common,  but  not  in  the  vicinity  of  Konnaoott.  The  bornite 

ind  chalcopyrlte  are  clearly  primary.   Their  contacta  show 
the  usual  mutual  outlines  which  are  believed  to  be  charac- 
teristic of  primary  relationships.  The  opinion  has  been  ex- 

1 
pressed,  that  in  .  certain  cases  the  bornite  is  secondary,  and 

should  be  expectej.  to  decrease  with  respect  to  the  chalcopyrite 
with  depth.      none  of  the  depoeits  aro  more  than  prospects 
thore  is  little  positive  evidence  of  changes  In  a  vertical 
direction,  but  the  microscopic  relations,  which  are  similar 
to  those  observed  in  ores  of  known  primary  character,  show 
that  this  view  is  unwarranted.  The  ore-minerals  corrode  the 
rock  minerals  or  the  gingue,  and  are  clearly  of  later  forma- 
tion,  calcite,  however,  is  an  exception,  being  in  part 
later  than  the  sulphides,  as  it  distinctly  breaka  thair  grains 
in  veins  in  uany  cases.   The  time  relation  between  the  jre- 
mlnerals  and  the  zeolites  is  not  known.  Both  bornite  and 
ohilcopyrito  were  observed  in  amygduxes,  usually  with  oalcite, 
with  which  the  sulphides  were  apparently  contemporaneous. 
Secondary  Alteration.  The  auount  of  surficial 
alteration  i«  small.  The  sulphides  are  themselves  exposed 
to  the  air  on  the  out-crops,  and  in  no  case  is  there  a  zone 
of  complete  oxidation,  ilalachite  uvi  liiaonite  in  varying 
•amounts  ^ro  always  present,  however.  Native  copper  and  cuprite 
oo<mr  in  miny  of  the  vcim-;     is  diseeminated  epecke  through- 
out the  rock  or  in  tho  araygdulea.   It  is  impossible  to  decide 

1.  Soffit,  ?.  H.  and  Capps,  S.  R.f  Bull.  :44£,  p.  95. 


238 


from  the  evidence  available,  whether  the  native  copper  was  de- 
rived from  pre-existing  sulphides  or  was  deposited  as  an  Initial 
constituent  of  the  ores.  The  cuprite  may  be  seen  under  the 
microscope  to  be  a  product  of  the  oxidation  of  the  native  copper. 

Deposits  noar  jthe  Bonanza  Mine.   Chaloocite  and 
covellite  are  not  abundant,  but  occur  in  small  amounts  in  all 
the  bornite  ores  studied.   In  nearly  all  cases,  the  ohaloooite 
is  clearly  a  replac  ... -ut  of  the  bornite,  but  occasionally  the 
minerals  are  associated  in  structures  which  are  difficult  to 
interpret.   In  i  specimen  from  a  vein  west  of  the  Bonanza  Mine, 
the  chalcocite  contains  smooth  blebs  of  bornite  with  shapes 
similar  to  blebs  of  the  graphic  structure.  Veins  of  ohalcocite 
in  the  bornite,  however,  of for  conclusive  proof  that  Ht  least 
part  of  the  chalcocite  is  of  replacement  origin.  The  chalooeite 
is  slightly  altered  to  malachite,  which  forms  series  of  straight 
parallel  veins  in  it,  revealing  the  coarse  grain,  and  crystal 
structure  of  the  sulphides.   Etching  with  nitric  acid  or  po- 
tassium cyanide  developes  fine  persistent  oraofcs,  oriented  in 
only  one  direction  In  each  grain,  and  parallel  to  the  direction 
of  the  malachite  veinlets.   The  structure  is  tint  commonly  ex- 
hibited by  orthorhomblc  chalcooite.  The  large  grains  of  ohal- 
cocite frequently  include  snaller  grains  of  the  bornite,  with 
no  apparent  interruption  of  the  crystal  structure  of  the  chal- 
cocite.  The  bornite  etches  with  its  u«ual  fine  grained  pattern, 
'and  shows  no  relation  to  the  orientation  of  tha  surrounding 
chaloocita. 

None  of  the  .  rospects  in  the  greenstone  near  Kermeoott 


neee  ecf 


,nJ: 


expose  more  thin  the  upper  ten  feet  or  so  of  the  deposits, 
consequently  no  positive  information  Is  available  concerning 
the  depth  of  oxidation  and  secondary  sulphides. 

Kuskulana  District.   In  the  Kuskulana  District, 
a  deposit  on  Nugget  Creek  is  exposed  to  a  depth  of  about  300  ft. 
and.  an  its  ores  are  vory  similar  to  the  material  in  the  green- 
stone near  Kenneoott,  they  offer  valuable  evidence  concerning 
the  nature  of  enrichment  in  the  greenstone. 

The  ore-bodies  at  Nugget  Greek  consist  of  lenses  of 
bornite,  with  subordinate  amounts  of  chaleopyrlte  and  chalcocite, 
along  a  shear-zone  in  the  Nikolai  greenstone.  The  bornite  and 
chalcopyrite  are  associated  in  structures,  commonly  regarded  as 
typical  or  primary  ores.   The  chalcocite,  which  Is  apparently 
as  abundant  on  the  lowest  level  as  near  the  surface,  is  of  two 
ages.   For  the  most  part  it  is  associated  with  the  bornite 
in  a  structure  identical  with  that  commonly  exhibited  by  the 
bornite  and  chalcopyrite  in  primary  deposits,  which  in  places 
merges  with  typical  graphic  structures  of  bornite  and  chal- 
cocite.  (  7i#a.l01,102).  There  is  distinct  evidence  of  attack 
on  the  bornite  grains  by  the  chalcooite,  however,  and  there  can 
be  little  doubt  that  part  of  the  chalcocite  is  of  replacement 

origin.   In  addition  to  the  chalcocite  in  these  relations  to 

thore  are 
the  bornite, A fine  veinlets  of  chaloocite  which  a  re  clearly  tn- 

pjsed  upon  ths  earlier  structures.  They  break  the  bornite  blebs, 
but  disappear  when  a  chalcocite  area  is  encountered.   In  some 
oases,  the  crack  followed  by  the  ve inlet  can  be  traced  acroea  the 


.(SOI-,I<UjnJ  too 


chaloooite  areas;   in  others,  it  is  apparently  of  submioro- 
sooplc  size.   (  Similar  relations  were  observed  at  the  Superior 
Prospect,  near  EiigelQ,  California;  (see  page  13S  ). 

The  fine  velnletB  of  chaleocite  are  dost  probably 
related  to  a  recent  surface  of  rapid  mechanical  degradation. 
As  the  ground  remains  frozen  throughout  the  year  except  at 
shallow  depths,  even  this  small  amount  of  enrichment  can  hard- 
ly have  taken  place  in  post-glacial  time.   It  is  possible, 
however,  that  it  may  have  occurred  in  an  interglacial  period, 
during  which  higher  temperatures  prevailed  than  those  it 
present. 

The  origin  to  be  assigned  to  the  earlier  chaloocite 
depends  upon  the  interpretation  of  the  graphic  structure  and 
the,  other  closely  associated  form.  The  latter  presents  most 
of  the  properties  of  the  graphic  structure  and  IB  surely  of 
similar  origin.   If  the  chalcooite  in  these  structures  is  a 
contemporaneous  intergrowth  with  the  bornite, slightly  modified 
by  subsequent  replacement,  the  chaloocite  is  of  primary 
origin;   if  the  ohaloocite  in  the  graphic  structure  is  entirely 
a  replaceir/ u,t  of  bornite  it  is  not  necessarily  primary,  and  the 
evidence  fpon other  districts  suggests  that  it  may  be  the 
product  of  deep  enrichment.   The  pre-voicanic  surface,  which 
probnbly  covered  this  region,  or  the  Inter  surface,  developed 
luring  the  pro-glacial  stream  erosion,  may  have  afforded  the 
necessary  conditions  for  deap  enrichment. 


j-fl-t 


Kotsina  District.  Still  further  to  the  west, 
on  Slliott  'ireeK.  in  the  Kotsina  District,  similar  bornite 
ore  occurs  in  the  greonotonen,  and  a  utudy  of  polished  spec- 
imens from  the  Mineral  King  claim  shov/s  a  very  interesting 
type  of  alteration  of  trio  bornite  to  oovellito,  ohalcooite  and 

'.Icopyrite.   The  bornite  possesses  a  medium  grained  struct- 
uro  which  13  emphasized  by  the  tendency  for  the  development 
of  covellito  and  ohalcooite  to  be  United  to  certain  grains. 
The  alteration  proceeds  by  means  of  a  net-v/orK  of  fine,  ir- 
regular veinlets,  almost  sub-microscopic  in  some  cases,  which 
are  about  equally  developed  in  any  part  of  a  single  grain. 
It  aeeua  ae  if  certain  areas  of  bornite  were  literally  Totting" 
to  covellite  and  chalcocitc.  The  covellite  is  the  earliest, 
an-'1  is  partially  altered  to  ehaloooite.   On  the  other  hand, 
oandB  or  areas  between  the  grains  appear  more  resistant.  The 
bornite  has  its  true  color,  and  where  it  is  penetrated  by 
chalcocite  or  covellite,  wall-defined  vainleta  are  found,  and 
not  the  fine  lacy  patterns  described  above.   The  resistant 
type  of  bornite,  hov/ever,  contains  numerous  i'ine  spines  of 
chalcopyrite,  distributed  in  a  lattice  pattern.   The  ehalco- 
pyrite  rarely  occurs  in  the  grains  which  are  altering  to  cov- 
ellite and  chalcocite.  The  chalcopyrite  is  earlier  than  the 
copper  sulphides,  for  their  veinlets  break  the  spines  In  some 
cases,  but  it  Is  closely  associated  v;ith  their  development. 

Similar  microscopic  relations  v:ith  the  except  ion  of  the 
confinement  of  the  replacing  sulphides  to  grains  or  zones  in  the 


bornite,  have  been  ccaacionly  observed  In  ores  of  known  second- 
ary origii..   Only  the  superficial  parts  of  the  deposits  are 
Jtnovm,  but  there  is  no  roason  to  doubt  that  these  changes  were 
r-uise"!  by  descending  solutions,  although  probably  from  a  pre- 
glacial  surface  rather  than  the  present  one,  for  there  can  be 
little  snrichiasnt  in  frozen  ground. 

Suraaary  _of  Important  Features.  The  most  striking 
and  significant  fact  related  to  the  mineralisation  in  the  green- 
stone is  the  wide  distribution  of  the  deposits,  in,l  their  great 
similarity  over  the  entire  region.   The  roofc  alteration,  the 

ganguo  minerals  and  the  primary  sulphides  show  little  variation 
throughout  the  district.  A  few  small  deposits  on  the  west  aide 
of  the  KUKfcul?.na  glacier  seem  somewhat  different,  as  pyrite  and 
magnetite  become  abundant,  and  garnet  is  prominent  in  the  gangue» 
but  in  these  cases  there  is  evidence  that  the  orsn  are  related 
to  an  intrusion  of  porphyry,  and  probably  are  of  very  different 
origin  than  the  usual  type  of  mineral  deposit  in  the  greenstone. 

The  asscoiT  ior.  $-C  ^loits,  quart;:  ruid  epidote  as  the 
chief  ?;nngue  minerals,  with  chlorite,  datolite  and  snail  amount* 
of  zeolites,  an^  bornite  and  ehalcopyrite  as  the  common  ore- 
minerals  IB  coercion  throughoiit  the  region,  and  offers  an  inter- 
esting comparison  to  oiher  ncnbers  of  thy  7/orld-wide  class  of 
copper  deposits  in  basic  lavau. 

11  size  of  the  depo'-i-,      their  irregular  bunchy 
character  are  points  of  mores  than  scientific  interest,  as  it 

'36  tho  outlook  for  the  development  of  notable  ors-bodies  under 
the  present  conditions  far  from  promising.  However,  the  con- 
trast v^ith  the  exceedingly  rich  ores  of  the  overlying  limestone 
nay  cause  one  to  underestimate  their  value. 


. 


ORE  PEP03ITS  IH  THE  CHITI3T01B  LIlAK 

The  ore-bodies  of  the  producing  mines  of  the  Kenneoott  District 
are  all  in  the  Chitistone  limestone.   The  mineralization  is 
especially  remarkable  for  the  great  size  of  the  sulphide  bodies, 
the  purity  of  ohaloooite  of  which  they  are  formed,  and  the 
almost  complete  absence  of  silica  in  the  veins  or  in  the  wall 
rook*  The  deposits  are  monotonous  from  the  standpoint  of  the 
mineralogist,  for  other  sulphideethan  ehaloooite  are  rare,  but 
in  the  few  places  in  whioh  a  variety  of  minerals  were  found  the 
complexity  of  their  relations  to  each  other  revealed  by 
micro BO oploal  study  of  polished  surfaces,  has  offered  numerous 
problems  of  absorbing  interest*  The  greatest  single  problem 
in  connection  with  the  Kenneoott  ores  le  the  secondary  or 
primary  origin  of  the  ohalcocite.  Consequently,  in  this  portion 
of  the  paper,  whioh  deals  with  description  rather  than  deduction 
and  speculation,  the  ores  will  be  discussed  without  the  usual 
separation  into  those  of  primary  origin  and  those  of  secondary 
origin. 

Geologic  Situation  of  Mines*  The  ore-bodies  .vorked  by  the 
Bonanza  and  Jumbo  Mines  are  situated  in  the  lower  beds  of  the 
Chitistone  limo;  tone,  at  the  bottom  of  the  dolomitio  horizon  and 
to  a  certain  extent  in  the  upper  beds  of  the  gray  limestone, 
described  on  page  217  •  The  ores  at  the  Erie  Liine  are  in  the 
lowest  beds  of  tha  formation,  immediately  above  the  greenstone 
contact,  but  at  the  Mother  Lode  Lline  on  the  eastern  elde  of  the 
ridge,  the  ores  now  exposed  are  about  1000  ft*  stratlgraphically 
above  the  contact* 


aJbocf 


Structural  Solutions.     The  forms  of  the  ore-bodies  in  the 

~~ 

two  great  mines  are  very  similar,  and  a  general  description 
may  be  given  which  applies  to  both.  The  ohaloocite,  which  is 
the  abundant  ore -mineral,  occurs  in  a  definite  fissure  zone 
with  a  vertical  or  a  high  dip.  In  places  it  widens  to  great 
massive  bunches,  but  in  general  the  tabular  form  is  maintained* 
The  bottom  of  the  ore  with  some  local  exceptions  is  against 
the  "flat  fault",  described  on  page  225  .  The  fault,  as  has 
been  stated,  is  parallel  to  the  bedding  or  is  elightly  steeper* 
In  the  Jumbo  ;<iine,  it  is  a  prominent  feature;  in  the  Bonanza 
.ine,  thero  is  evidence  of  its  existence,  but  it  does  not 
form  as  definite  a  lower  limit  for  the  ore.  The  intersection 
of  the  main  vertical  fissure  zone  and  the  flat  fault  is  a 
particularly  favorable  locus  for  the  mineralization.  Tha 
largest  ore-bodies  in  both  mines  are  formed  by  the  widening 
of  the  vertical  veins  along  this  horizon.  (Fig.  35  ) 

The  vertical  fractures  as  well  as  the  ore  are  strongest 
near  the  flat  fault ,  and  tend  to  die  out ,  as  one  passes  higher 
strati graphic ally.  There  is  no  evidence  of  notable  movement 
along  them.    In  a  few  places, where  the  mining  operations 
have  penetrated  the  ground  beneath  the  fault  flat,  the 
vertical  fiesurinrr  apparently  dies  out  or  decreases  very 
greatly.  There  IB  a  strong  suggestion  from  the  relationship 
that  the  vertical  fractures  and  the  flat  fault  were  of  con- 
temporaneous origin.  As  has  been  mentioned,  there  is  evidence 


'ice ».     . 


suggesting  rather  violent  thrust  movements  along  the  plane  of 
the  flat  fault.  Vertical  breaks,  relieving  differential 
stresses,  would  be  vary  provable  aooompaniraents  of  sttoh  move- 
ment, and  the  vertical  fractures,  .vhioh  have  afforded  tha  ohief 
chancel  ways  for  the*  ore -forming  solutions  naay  have  been  formed 
in  this  way,  but  the  failure  of  the  vertical  fraotures  to 
oontlnua  below  the  flat  fault  may  possibly  be  due  merely  to 
displace  i«nt  along  it* 

Cross-breaks  are  numerous*  Several  aeries  were  observed 
and  studied*  For  the  most  part,  they  are  pro-ore,  >>ut  in  the 
few  oases*  whera  thoro  ie  post-ore  jnovonont  it  admits  only 
to  a  few  feet*  In:?  ore  tends  to  rru.ke  out  along  the  earlier 
oross-breaks  to  a  slight  degree,  but  rarely  extends  far  from 
the  main  fracture  zone  although  frequently  far  enough  to  cause 
a  local  swelling  of  the  vein*   In  parts  of  both  mines,  more 
then  one  vertical  fissure  is  mineralized,  and  in  these  localities, 
good  ore-bodies  are  common  along  cross-breaks  between  the  two 
veins.  Bedding  planes  have  played  a  similar  role  in  directing 
the  ore-solutions  ~»ut  are  less  prominent  than  the  cross  fractures* 
Zonae  of  fine  ohalcocite  ve inlets  are  common  in  certain  portions 
of  the  ore,  but  disseminated  sulphide  bodies  do  not  oocur.  In 

peneral,  the  ore  chows  a  marked  dependence  upon  the  main 

i!j  only  slightly  modified 

vertical  or  nearly  vertical  fracture  syctetn,  and  by  the  other 

/A 

structural  features  with  the  exception  of  the  flat  fault,  vvhioh 
usually  forms  the  bottom  of  the  ore*  The  relations  are 


'- 


• 


generalized  In  tho  idoalisoo  sections  shown  "below. 


/  on  a  i  tuJind!    section 


Cross     fractures 
-N  Cf"s   fracture 


Trans  fers  e 
sec  tton 


\Ho    sca/e) 


35.      Idealized  longitudinal  and  transverse  sections 
of  the  ore -"bodies  of  the  Xonnooott  -..Lines. 

The  largest  ore-bodies  aro  in  the  lower  portions  of  tho 
vertical    ;  cure  zone  near  the  flat  fault.  Passing  upward, 
strut! graphic; ally,  tho  mineralization  becomes  v?eaker,  and  the 
line  representing  the  upper  limit  of  finable  ore  is,  in  a 
broad  ,vay,  strikingly  parallel  with  the  dip  of  the  flat  fault 
and  trie  bedding  of  the  limestones.   (Fig.  35   ) 


- 


At  tho  Erio  :,:ine.  tho  ores  are  also  formed  along 
vertical  fissures,  "but  they  differ  from  the  greater  deposits 
In  that  the  fractures  cross  the  greenstone-  limestone  contact t 
and  ninortlisatlon  occurs  ttlong  the  "breaks  in  both  rooks. 
The  veins  in  the  groenutone  are  thin  and  of  no  commercial 
vc.lue,  ..'hilo  those  in  the  limestone  aro  wider  and  promising, 
Tho  ores  in  the  limestone  are  in  the  lower  siliceous  beds, 
or  In  the  grey  limestone,  at  least  as  far  as  they  had  "been 
exposed  at  the  ti^e  o'"  our  work. 

The  Mother  Lode  mine  depends  on  ores  formed  along  a 
system  of  eteep  or  vertical  fractures  in  the  dolomltio 
limertone,  far  above  the  horizon  of  the  other  oro-bodies  of 
the  district.  The  fiseuring  ie  distinct,  "nit  is:?  less  intense 
than  at  the  Bonanza  Mine.  The  lower  limit  of  their  ore  is  not 
known  at  present • 

Gun,-;ue  Minerals  and  rock-alteration*    One  of  the  most 
remarkable  features  of  the  ores  in  the  Chitistone  limestone 
is  the  great  simplicity  of  the  gungue  minerals,  and  the  lac'k 
of  siliclflcatlon  of  the  wall  rook.  The  wall  rook  ie  commonly 
reoryi tallized  along  the  voins,  forming  ooarse,  grained  maesos 
of  calclte  and  dolonlte,  but  in  many  places,  stringers  of 
ohaloooite  ooour  directly  in  limestone  of  normal  prein,  which 
even  under  the  microscope,  shows  no  alteration  of  any  sort. 
Only  a  few  umall  Drains  of  quartz  were  observed  in  a  study  of 
a  large  number  of  thin  sections.  lio  silicates  .vere  detected 
either  in  tho  field  or  in  the  course  of  tho  laboratory  study  of 
the  material  collected. 


The  reorystallizatlon  of  the  limestone  alon?  the  veins, 
and  the  deposition  of  new  ealcite,  or  dolomite  tie  vein-filling, 
is  probably  a  phase  of  the  mineralization,  "but  evidently  pre- 
ceded the  development  of  the  sulphides  in  some  plaoee  for  In 
many  oases  tho  coarse  crystals  of  the  carbonates  were  observed 
to  have  been  Attacked  along  thalr  cleavages  or  othor  lines 
of  structural  weakness  by  voinlets  of  ohalaooito.  Tho  formation 
of  the  carbonates,  however,  extended  considerably  beyond  the 
limits  of  the  oro,  and  consequently  the  continuation  of  tho 
fraoturee  is  marked  by  the  veins  of  carbonates.  Barren  veins 
of  these  minerals  are  common  in  various  parts  of  the  Chltlstone 
formation,  however,  as  would  be  expected  In  a  limestone  country. 
The  gangue  minerals  in  the  veins  at  the  Eria  Mine  show  an 
interesting  dependence  on  the  wall  rook*  In  the  greenstone, 
quartz  predominates  and  oalclte  is  subordinate.  Tho  same 
fracture,  followed  Into  the  limestone  contains  a  gangue  com- 
posed only  of  caloite  and  dolomite  r.ith  no  quartz. 

)ro-!ninerals»   Relative  abundance.  The  ore-minerals, 
with  the  exception  of  obvious  products  of  oxidation,  are  as 
follows,  lifted  in  the  order  of  their  quantitative  importance :- 
(1)  chaloocito,  (~)  covelllte,  (3)  bornite,  (4)  enargite, 
(T)  chaloopyrite,  (6)  lusonltef? ) ,  (7)  tennantlta,  (8)  pyrlte, 
(9)  sphalerite,  and  (10)  galBna.   Of  theso,  chalcocite  is  by 
far  the  most  important.  In  tho  flold,  we  estimated  that  It 
formed  90  -  95$  of  tho  sulphide  ore,  and  no  modification  of 


this  figure  seems  necessary  from  the  reiralts  of  laboi'atoi'y 
work.  Co/ellite  is  fairly  abundant,  compared  v/ith  the  ro- 
taaitfing  minerals  ,  and  when  ite  wide -spread  distribution  in 
fine  threads  and  lathe  throughout  the  ohalcocite  is  tukan 
into  account,  it  is  not  unlikely  that  it  forms  from  3  to  5/S 
of  the  ore -minerals.  The  remaining  sulphides  probably  con- 
stitute lesci  than  £  of  tho  ore.   *f  these,  bo.-nite  is  the 
most  generally  distributed.  Grains  over  an  inch  in  diameter 
are  vory  rare  .however,  but  e;aall  ,-  ec  :d  the  size  of  a 
millemeter  or  less  in  diameter  are  oorrunon.  Locally,  enarglte 
is  1  .poi'tant.  In  the  Bonanza  -.Una  It  is  probably  more 
abundant  than  the  bornite,  nut  for  tho  district  as  a  whole, 
bornite  undoubtedly  forces  it  into  fourth  plaoe.  Chaloopyrite 
occurs  megascopioally  only  in  e  few  eraall  bunches,  and  is  so 
rare,  that  it  has  been  mistaken  for  £0ld  by  the  miners* 
Lusonite  (?)  and  tennantite  are  not  uncommon  under  the 
microscope,  "hut  were  not  observed  in  the  field.  Pyrite  IB 
noticably  absent  in  most  parts  of  the  deposits,  and  its  total 
amount  is  exoeeollngly  small.  Sphalerite  and  galena  are 
microscopic  rarities. 

Chalcoolte.   Xwo  distinct  types  of  chalcocite  occur  in 
the  deposits  in  the  Chitistone  1"     :>ne,  which  «re  tormed  for 
convenience  in  the  field,  atooly^ j.:halcocite  and  crystalline 
phaloooite,  roepeotively*  Tha  Tormer  is  a  compact  massive 


mineral,   '. th  no  ,   conchoids!  fracture,  and  a 

metallic  steady  lur.tor.  The  crystalline  chslaocite  appears  to 
have  &  .jrarmlar  structure  on  a  fracture -surface ,  as  If  the 
material  /ore  composed  of  an  aggregate  of  individual  crystals 
of  about  the  coarseness  of  a  medium-drained  marblo.  Uo 
crystal  faces  were  observed  on  any  of  the  grains,  however, 
"but  the  crystalline  appearance  1$  largely  duo  to  an  imperfect 
cleavage. 

The  "stoel;/  ohaleoclto"  is  most  abundant  in  the  Bonanza 
Mine,  the  "crystalline"  in  the  Jumbo  Mine.  The  tv.'o  types 
ooumonly  grade  into  each  other  -Tlthou!;  sharp  Boundaries,  but 
in  &  few  rare  instances,  "bands  o~  the  coarse  nu-terial  were 
found  ir.  the  fino  an,1  visa  versa. 

The  distinction  between  the  t^o  types  IB  lost  oompl^t 
on  the  freshly  polished  surface,  for  both  assume  the  same  even 
bluish  tint  in  reflected  llrht.   In  the  course  of  a  few  hours, 
however,  a  slight  tarnish  usually  forms,  revealing  the  grain 
of  the  material,  end  often  the  cryBtallorraphlc  structure. 
In  the  coarser  grains,  a  pattern  of  triangles  or  rectangles  is 
formed  by  lighter  b&nde  or  Strips  cutting  areas  v;hioh  tarnish 
to  f.  deeper  blue.   (Fig.  119  )   Tho  structure  is  identical 
v/ith  the  lattice  structure,  previously  described  (page  I?1*- > 
figures  lM-6,152). 

-^hin  the  polif<:hed  eurfacee  with  dilute  nitric  acid 
and  ,ith  dilute  potaoaiuni  cyanide  solution  usually  reveals  the 
In  of  the  mineral.  The  crystalline  chalcocite  appears  com- 


Jt     el: 


sei, 


251 


posed  of  large  grains  oe.-nentod  by  smaller.  (?!#.  10?  )• 
Both  large  and  small  grains  develope  lattice  patterns  when 
©tchod,  the  smaller  ones  imperfectly  as  vould  "bo  azpeote  . 

c-o^el orally  the  pmallT  rains  develops  only  one  set  of 
strone  lines,  similrr  to  tha  orthoryor-tbio  etoh-cleave.g<i  of 
chnlcocite,  'but  the  structure  is  very  subordinate*  The 
oteely  chalcocite,  when  etched  with  nitric  acid,  yields 
Irregular  patterns  of  short  curved  or  stri •ipJvi  cracfcs  with 
no  definite  oriontation  -7h5oh  produces  a  surface  strikingly 
similar  to  that  of  crackled  porcelain*  (Fig.  105  )  Potassium 
cyanide  solution  on_tho  sc^o  mr:terlRl  brings  out  the  fins  grain 
of  the  minerBl,  end  here  and  there  develops  an  imperfect  lattice 
pattern  vithln  the  prsin  ^ cund&riea. 

In  other  casee,  however,  both  in  steely  end  crystalline 
chelcocite,  tho  original  prain  of  the  material  seeas  lost* 
The  pollyhod  surfeoe  IP  finely  mottled  with  tiny  dote  of  light 
^lue  and  white,  oloeoly  pr.c*ked,  with  here  end  there  e  larger 
"blulah  area.    '  ^hlng  with  the  reagents  merely  develops  a 
rough  solution  surface  vrlth  no  apparent  structure  in  the 
mottled  material.  The  bluieh  areas  see-n  more  resistant  to 
cyanide,  and  usually  yiald  &  lattice  pattern*  ^ttch  of  the 
crystalline  chulcooite  of  the  lerga  ore-body  of  the  Jumbo 
:'lne  Ifi  of  this  sort* 

Another  structure  of  interest  in  the  chaloooite,  although 
far  leso  abundant  than  thoee  previously  describes,  ie  a 
peouliar  oonoontric  'orra  ac;  uiaen  by  the  miner;  1,  in  small, 
localized  spots  in  tho  ore*  Where  developed  on  rather  a  fine 


252 


scale,  the  ohalcooite  appears  as  if  made  up  of  an  aggregate 
of  BftaH  pd&*ll)M  -rains;  the  finld  name,  pebb ly  chalc o o  i  t  e «  • 
Is  descriptive  and  .-/ill  be  used  in  reforrlnr  to  it.  ?n  the 
polished  {vr-f&ce,  thr»  structure  >f  •;?.  -  -.ineral  is  emphasized 
by  '.;o  no  en  trie  clacks  in  the  individual  grains,  or  by 
scalloped  YQ inlet s       "nates.  Small  open  spaces  rarely 
exceeding  6  in.  in  their  -roatest  dimensions  hsve  been  ob- 
served in  the  ore,  and  in  a  few,  the  chalcocits  of  the  walls 
possesses  a  -.veil  developed  mesmlllary  surface.  Tho  chalcocite, 
:.ir  the  mineral  v/hi ch  it  replaces,  may  most  reasonably  be  inter- 
preted as  en  open  space  flllinr-. 

The  various  structures  of  the  chalcoclte  are  well  brought 
out  by  voinlet     oarlonatss  or  other  alteration  products  which 
are  abundant  In  all  pt-rte  of  the  ore-bodies.  The  lattice  structure 
ie  particularly  wejl  shown  in  its  various  degrees  of  fineness 
(figures  103  and  116}  As  has  been  -nentioned,  tha  concentric 
structure  in  likewise  revealed  by  the  carbonate  voinlets,  and 
much  more  clearly  than  it  oan  be  shown  by  etching. 

' th  the  esoeptior  of  a  certain  fraction  of  the  oovellite, 
whicsh  is  plainly  associatet;  ,vith  tho  carbonates,  the  ohalcocite 
is  the  latoet  sulphide  to  form.  In  the  ease  of  every  other 
miner-  1  ,'n  tho  lint  on  page 303  ,  there  is  oloar  evidence  that  it 
has  ru'fored  partial  replacement  by  ohaloocite,  In  some  parts 
of  the  deposit  at  least.  The  relative  ease  of  repl     at  of 
the  earlier  minerals  by  ah.ilcooite  varies  notably  as  is  commonly 
found  ta  ])0  the  case  ii;  other  c<;jnps.  Bornite,  us  usual,  nroas 


'•    . 


£01 


most  reedily  v'ith  ohalcopyrite  a  poor  second.   :  nargrite  and 
tennejitite  ero  less  easily  replaced,  tut  not  ae  resistant  as 
the  luzonitp' ":  ;  /hieh  often  remains  as  fine  voinlets  or  rings, 
partir  j.ly  corroded,  in  the  chaloooite  after  all  neighboring 
material  had  "been  consumed* 

The  relations  between  tho  ohalcooite  and  other  sulphides 
will  "ho  de  so  ribs  d  in  great or  detail  in  the  paragraphs  dealing 
v7j.th  the  other  rninerGls. 

Oureful  chonioal  studios  of  chaloocita  from  tha  Bonanza 
and  Jumbo  'linos  have  been  made  in  the  Geophysical  Laboratory 

in  Washington.  Tv.-o  Analyses  ana  specific  gravity  determinations 

1 
have  been  published  ,  and  are  as  follows: 

Specific  rrr.vity    Per  oent  Per  oont  Per  oont  Per  cent  Total 
at  £5  Gu        3       ye     Si  Og 

5.610  77.99      21.48    0.26     0.13    99,86 

5. GOG  77.56      £1.55    0.55     0.18    99.84 

The  analyses  of  ohaloooite  collected  during  our  work  in 

1915  are  in  striking  aocord  with  the  earlier  results.  Material 

2 

from  tho  Jumbo  liines  yielded  the  following  values: - 


Specimen 

Steely  glance 

Crystalline  glance 

Cu 

77.90 

77,38 

Ag 

.07 

.09 

s 

.06 

.04 

Pe 

.17 

.17 

S 

21.33 

21.48 

Insoluble  G».in?ruo 

.10 

•Of 

Ca  0 

.11 

D 

.02 

U  )£  (?) 

.11 

1.  i: .  Poenjak,  L. T.Allen,  and  H.ii.  Merwin,  Loo.oit.,p.  508. 

2.  K.T.  Allen,  private  communication  Deo.,  1915. 


Coarse  grained  crystalline  glance  from  the  Bonanza  ..:ine  was 
also  found  to  have  almost  the  same  composition  that  the  other 
chalcooites  from  the  region  have.  Dr.  Allen  writes:"!  find 
the  copper  to  "he  78.007.,  the  silver  O.)5,w,  the  insoluble 
gangue  0.03-/i.  It  alBO  seems  to  contain  a  trace  of  arsenic. 
It  seems  to  me  that  the  remarkable  uniformity  in  composition 
of  such  a  great  body  of  ore  is  very  significant  as  to  the 
uniformity  in  the  chemical  cunditiona  of  formation." 

The  chalcooitee  Bent  to  the  Geophysical  Laboratory  were 
selected  after  iuinsraloArar.nio  examination  au  examples  of  ae 
pure  material  as  could  bo  obtained.   Vary  minute  grains  of 
bornite  were  observed  under  tho  highsst  magnification,  and  their 
presence  probably  HO counts  for  the  iron  in  the  analyoas.  In 
sorae  specimens,  especially  t,ho  cteely  glance,  .fine  grains  of 
luzonlte  (?)  occur,  and  possibly  a  little  onargite .  The  arsenic 
is  due  to  the  presence  of  these  minerals  .vithout  doubt.  A  little 
oovellite  was  seen  as  very  fine  threads  in  tho  chalcocite,  but 
almost  negligible  in  amount.  The  surfaces  of  the  chalcocite 
exhibited  the  faint  bluish  mottling  common  in  the  Kennecott  ore. 

Concerning  specific  gravity  determinations  on  the  new 

~1 
material,  Dr.  Allen  -/rites:   "I  ho.vo  determines  the  gravity  of 

both  Kn  1166  (crystalline  chalcocito      ihe  Bonanza  Mines) 
and  KJl  ISOOz  ( crystalline  chalcocite  from  the  Jumbo  ...ine)  by 
both  the  Archimedes  and  the  piiknoaoter  methods,  anu  find,  con- 
trary to  our  expectations,  a  similar  porosity  in  both.   The 

1.   Private  communication,   (Jan.  1916). 


255 


specimen  Xn  1200s  however  io  still  nearly    li  htor  than  a 
solution  of  Cu  S  in  Ou:;  o  should  "bo.  We  can  at  present 
account  "or  this?  only  by  supposing  the  specimen  to  possess 
that  muo'h  pore  space  in  capillaries  too  fine  for  water  to 
penetrate  under  our  conditions.  The  results  are  aa  follows :- 

Kn.  1166 

a.  Archimedes  T.othod  at  25         .'"91 

b.  likno-oter     "   "   "        5.G28 
Kn  laOOs 

a.  .-rohi'no:      !:hod  at  25        3.473 

b.  Pllcnoraotor    w    "  "        5.531 
Covollita.   Covolllt  -  >f  t-,vo  distinct  ageo  may  "be  recog- 
nised with  oertclnty  in  the  7nnneoott  orac?.  Part  of  the 
jovelllte  ic  oarlier-  than  the  ohalcooite,  ancl  seams  aaaooiated 

Va  the  primary  rainer&lizatlon.  An  even  ^r--.- -tor  amount,  however. 
Is  conclusively  shown,  ^oth  "by  field  and  microscopical  ovidenoe, 

• 

to  "be  the  first  product  of  tho  o  7.1  Act  ion  of  the  ohalcooite» 
this  part  of  th-3  oovellite  ia  olo--oly  assocjiatec-  in  its 
davol^r  iiont  and  distribution  with  malachite,  azurite  and 
litr.ani^e,  it  .ill  "be  r:.ncicloro "  on  the  pages  dealing  with 
oxidation.   The  earlier  oovellito  occasionally  oocurs  as 
broad  crystalline  "oar.de  through,  ^he  ore,  "but  it  is  ooraraonect 

hort  lathe  ecatterad  in  the  ahalcocite. 

Locally,    the  "bloct  crystdlr-  o   abundt-nt  encup-h  tj  resemble 

tho   distribution    if  he   in  a   filifbesi    (rJ-j-  109      )   but 

ordinarily  they  are  very  small  and  fora  a  subordinate  pt.rt   of  the 


256 


total  sulphide.  Pine  threads  of  covellite  are  almost  always 
present,  even  in  the  purest  ahalcocite. 

Bands,  an  inch  or  so  wide,  of  ooarsely  crystalline 
covellite  penetrate  the  massive  ehalcocite  in  a  vein-like 
manner  in  several  plaoes  but  when  studied  under  the  microscope, 
the  chalcooite  is  olearly  shown  to  be  later,  as  it  forms 
fine  veinlets  along'  the  boundaries  of  the  oovellite  crystals 
and  even  breaks  across  them.  (Pig. Ill)   The  oovellite  bands 
undoubtedly  formed  as  veinlets  in  an  earlier  sulphide*  In 
several  oases,  the  chaloooite  near  the  oovellite  contains 
residues  of  bornite,  and  it  is  most  probable  that  in  many 
oases  at  least,  the  oovellite  was  developed  in  that  mineral. 
The  oovellite  crystals  have  thoir  longer  axes  perpendicular 
to  the  course  of  the  vein,  and  on  the  polished  surface,  they 
offer  a  striking  exhibition  of  the  ploocroio  properties  of  the 
mineral,  varying  from  a  deep  blue  parallel  to  the  plane  of 
polarization  to  pale  bluish  .vhite  -vhan  a*  right  angles. 

In  a  f e v  specimens,  believed, when  collected,  to  be  en- 
tirely composed  of  oovellite  the  microscope  revealed  a  complex 
a.~p-regate  of  blunt  covellite  laths,  with  chaloopyrite  and 
bornite  filling  the  small  angular  spaces  between  them  (Pig. 110  ) 
a  relation  very  similar  to  the  feldspar  laths  and  the  ferromag- 
nesian  minerals  in  a  meoium  grained  diabase.  The  covollite 
laths  are  sh&rply  bounded,  and  broak  across  bornite-  chaloopyrite 
contacts  without  the  slightost  ohange.  The  ordinary  criteria 
of  sequence  among  minerals  of  igneous  rocks,  if  applied  here 


257 


would  put  the  oovollito  earlier  than  the  bornlte  and  chalcopyrite 
for  the  latter  apparently  moulded  themselves  about  the  oovellite 
crystals*  On  the  other  hand  the  usual  ae<  uenoe  elsewhere  in 
the  deposit  argues  that  the  oovellite  is  later  than  the  iron 
sulphides.  The  disregard  of  the  oovellite  for  bornite  or 
chalcopyrite  grains  is  however  difficult  to  explain  on  this 
basis,  for  under  ordinary  conditions  of  replacement,  the 
greater  ease  of  the  alteration  of  bornite  to  oovellite 
than  of  ohaloopyrite  to  covellite  is  very  definitely  shown. 
It  is  possible  however  that  the  ohaloopyrite  in  these 
associations  is  a  replacement  of  bornite,  and  developer  with 
the  covellite,  but  the  blooky  grains  in  which  it  occurs  are 
unlike  the  forms  commonly  assumed  by  chaloopyrite  of  such 
origin, 

Bornite.  In  nearly  all  parts  of  the  deposit,  small  amounts 
of  bornite  are  found,  but  it  is  less  abundant  in  the  crystalline 
type  of  chalcocite,  and  most  abundant  in  steely  glance  in  the 
Erie  Mine  and  certain  parts  of  the  Bonanza  Mine.  It  is  most 
common  in  small  grains  and  patches  v/ith  rather  smooth  outlines 
in  chalcoolte.  They  rarely  exceed  an  inch  diameter,  and  are 
most  abundant  as  tiny  speoks  of  microscopic  size.  The  larger 
rraina  are  often  broken  by  veinlets  of  ohalcooite  which  afford 
clear  evidence  of  replacement.   (Fig. 11#  )  The  veinlets  usually 
follow  cracks  in  tha  bornite,  which  may  contain  malachite  or 
limonite  in  some  cases.  The  oxidized  material,  however,  rarely 
extends  beyond  the  limits  of  the  bornite,  and  only  in  the  case 
of  the  larger  voins  does  the  crack  continue  on  into  the  chalcooite 


field.   (Pig.  117  ).  The  malachite  voinlets  in  regular 
lattice  patterns  in  the  chalooaite  either  do  not  extend  into 
the  bornite  or  else  follow  curving  oraoks  in  it  v/ithout  yield- 
ing definite  patterns.   (Fig. 120).  The  bornite  etches  .vith 
its  usual  fine-grained  lines  "but  their  orientation  rarely 
agrees  with  the  structural  directions  of  the  surrounding 
ohaloooite. 

Definite  lattice  patterns  between  bornite  and  ohaloocite, 
similar  to  those  observed  in  Bisbee  ores  (page  >7rf  )  do  not 
occur  at  Kenneoott,  but  in  some  pieces,  the  distribution  of 
faint  bluish  residues  and  the  pattern  of  blue  and  white 
chalcooite  strongly  suggest  the  last  stages  of  replacements  of 
the  lattice  type.  This  impression  gains  support  from  the 
occurrence  of  residual  spines  or  more  correctly  plates  of 
bornite  in  ohaloooite,  in  symmetrical  orientation  to  the  pattern 
in  the  ohaloooite,  which  is  strikingly  similar  to  the  association 
common  in  bornite-chaloocite  ores  from  Butte  and  Bisbee,  where 
the  lattice  structure  is  important.  In  places  where  the  inter- 
secting spines  are  abundant,  the  relations  resemble  forms 
assumed  by  Certain  Intergrovrbhs  of  minerals  (magnetite  and 
ilmenite  for  example)  and  of  artificial  products  (ohalcopyrite 
and  bornite;  certain  iron  carbon  compounds  in  steel.)  Careful 
study,  however,  of  the  contacts  invariably  yieidi  evidence 
of  ohalcocito  replacement  of  "hornite,  and  establishes  the 
residual  nature  of  the  spines. 


teli 


The  lattice  structure  IB  shown  also  by  spines  of  ohalcopyrite 
in  bornite,  similar  to  the  relations  described  previously  at 
Engels,  LaFleur  i;t..  Copper  Lit.,  and  Seven  Devils.  The 
ohaloopyrite  in  many  oases  eho./s  the  same  dependence  on  chalooclte 
voinlets  in  the  bornite  as  has  been  noted  in  the  other  deposits, 
but  the  intense  development  of  ohalcocite  in  the  Kennecott  ores 
usually  masks  the  evidence  of  incipient  alteration.  However, 
it  is  clear  here,  as  in  the  other  camps  that  the  ohaloopyrite 
is  a  product  of  the  reactions  by  which  the  ohaloooite  was  pro- 
duced from  bornito.  In  places  the  change  becomes  of  quantitative 
importance,  and  the  intersecting  spines  in  the  fine  grill  become 
so  numerous  as  to  yield  an  almost  solid  mass  of  chalcopyrite. 
(figures  1*4    ) 

Certain  narrow  strips  of  bornite  often  with  sharp  offsets 
seem  lees  subject  to  alteration  to  ohaloopyrite,  and  usually 
remain  untouched.   They  greatly  resemble  voinlets  of  bornite 
in  chaloopyrite,  and  in  the  extreme  oases  of  ohalcopyrite  formation 
it  is  difficult  to  escape  from  the  interpretation  that  chalcopyrit® 
was  the  earlier  mineral.  However,  by  tracing  the  relations  from 
the  incipient  stages,  where  a  faint  band  of  chaloopyrite  spines 
is  forming  along  the  margins  of  these  strips  of  resistant  bornite 
to  the  final  stages,  where  the  main  field  of  bornite  has  been 
completely  altered  and  only  the  strips  remain, the  sequence  of 
bornite  to  chalcopyrite  may  be  firmly  established. 

By  these  changes,  intimate  mixtures  of  hornite  and 
ohalcopyrite  aro  produced.  Prom  them,  the  bornite  is  extracted 


260 


in  many  cases  by  alteration  to  eovellite  or  less  commonly  to 
chalcooite.    Extremely  complex  patterns  of  covellite,  in 
fine  lines,  specks  or  vo inlets  in  chalcopyrite  result,  BO 
intricately  end  finely  spaced  that  under  low  magnificat lone, 
the  surface  appears  almost  as  if  it  were  a  single  mineral 
with  a  peoular  shade  of  yellow. 

No  typical  graphic  ctruotures  between  bornite  and  ohal- 
oooite  were  observed  in  the  Konnecott  ores,  but  in  some  cases, 
in  chalcooite  clearly  of  replacement  origin,  residues  of  bornite 
assume  forms  very  similar  to  the  shapes  of  the  blebs  in 
graphic  areas.  Ihese  sub-graphic  structures,  as  they  may  be 
termed,  are  usually  on  the  edge  of  bornite  areas,  and  are  part 
of  broad  borders  or  margins  >f  chalcocite  which  are  ..ithout 
question  a  replacement  of  tho  bornite. 

In  the  preceding  paragraphs,  the  results  of  various 
alterations  of  the  bornite  have  been  considered.  The  relations 
of  the  bornite  to  contemporaneous  or  earlier  sulphides  remain 
to  be  considered,  and  will  be  treated  under  the  headings  of  the 
following  minerals. 

Bornite  continued  to  form  under  the  conditions  which  pro- 
duced the  earliest  covel.lite,  if  the  evidences  presentee;  on 
pages  256-257   is  to  be  accepted.  This  weak  continuation  or 
renewal  of  bornite  formation  is  shown  by  fine  voinlets,  which 
break  later  structures,  and  later  linerala  as  luzonite  or^ 
rarely,  the  covollite. 


' 
„  f  -  r  -vrv.  • 

• 

' 

' 
. 

• 

• 


. 


g*lc 
| 

. 
. 

- 


261 


A  puzzling  occurrence  of  bornite  In  nodules  about  the 
size  and  shape  of  pigeon's  eggs  wae  noted  near  the  Erie  Mine. 
The  nodules  are  in  the  lo.ver  siliceous  limoctone,  and  apparently 
have  no  connection  with  the  veins  or  ore -bodies  either  in  the 
limestone  or  in  the  underlying  greenstone.  Ho  structure  could 
be  observed  in  the  bornite  v/hich  would  surest  a  replacement  of 
a  fossil  or  of  some  impurity  in  the  rook. 

In  one  nodule  studied  microscopically  the  bornite  is  cut 
by  numerous  vo inlets.  The  centers  of  the  ve inlets  are  usually 
malachite  and  oalcite  with  llmonite  on  the  sides.  They  are 
bordered  by  thin  margins  of  oovellite  which  in  turn  is  succeeded 
inward  by  narrow  bands  of  chaloocite.  Many  grains  of  the 
bornite  contain  fine  lattices  of  ohaloopyrite  spines  which  in 
plaoes  range  down  to  almost  submiorosoopic  size.  Certain  bornite 
grains,  with  unusual  yellowish  tints,  are  most  probably  filled 
v. ith  chaloopyrite  of  this  character,  too  fino  to  be  detected  by 
our  highest  magnifications. 

The  nodule  presents  a  v/onde fully  complete  record  of  the 
alteration  of  bornite.  The  slight  stains  on  the  surrounding 
limestone  indicate  that  little  except  water  and  oxygen  and 
calcium  carbonate  have  been  added  to  the  nodule  and  that  little 
has  been  removed.  Consequently,  the  chief  work  of  the  oxidizing 
process  hae  been  to  rework  the  mineral  combinations,  with 
little  addition  or  subtraction  of  material.  As  the  alteration 
advances  into  the  nodule  along:  cracks  and  seams,  the  copper  of  the 
bornite  is  transposed  into  ^halcocite.  The  iron  liberated  by 


'II 


• 


262 


this  reaction  is  In  part  foroed  ahead  of  the  main  alteration 
and  concentrated  in  the  protected  interior  producing  the 
lattice  of  ohaloopyrite,  end  in  part  finds  its  way  into  the 
principal  channels,  where,  vith  the  acidity  of  the  solutions 
re.  ucod  by  calcium  carbonate  from  the  limestone,  it  is 
oxidized  and  hydrolyzed  to  lirnonite,  tnd  therefore  little  iron 
escapee  beyond  the  limits  of  the  nodule.  The  continuing  attack 
of  oxidation  yields  covellite  from  the  chaloocite  with 
malachite  as  the  final  product. 

The  nodule  affords  convincing  evidence  of  the  details 
of  the  alteration  of  bornite  to  secondary  pulphldeo.  It  is 
not  u  case  of  enrichment  if  the  entire  module  is  considered 
for  no  copper  has  beon  add  eel,  but  its  processes  are  identical 

h  ;.hose  v/hiah  take  plaoe  in  the  enrichment  of  larger 
ore-bodies.   The  sequence  of  alteration  products  is  not 
simple  but  it  may  be  illustrated  fairly  completely  by  the 
diagram  below.  The  alteration  of  one  mineral  to  the  other  is 
in  the  direction  o?  tho  arrows. 


Bornite. 


Litnonite    and    Malachite 

:  i  :.36.£iagram  of  mineral  sequence  in  bornito  nodule  from 
limestone  near  Erie  Mine. 


inargite.  Enargite  ooours  abundantly  in  coarse  crystalline 
masses  in  a  few  places  in  the  Bonanza  Mine,  and  in  small  amounts 
throughout  the  ore,  espeoially  in  the  steely  ohaloooite.  None 
was  observed  megascopioally  in  the  Jumbo  ore.  In  the  crystalline 
ehaloooito,  which  is  the  most  important  type  in  this  ore-body, 
there  is  little  even  under  the  microscope.   On  the  whole,  it 
is  far  less  abundant  than  bornite,  although  a  considerable  mass 
of  it  was  exposed  on  the  100 *L  of  the  Bonanza  LJine,  where  the 
mineral  was  first  observed  by  Dr.  Bateman. 

The  enargite  is  usually  associated  with  ehalcocite,  which 
replaces  it,  yielding  a  peculiar  brecoiated  structure,  v/ith 
angular  fragments  of  the  enargite  set  in  r  cement  of  blue 
ehalcocite.   In  ^orae  cases  etching  develops  a  well-defined 
triangular  pattern  in  the  chalcocite.  The  enargite  apparently 
alters  lose  easily  than  bornite,  and  the  replacement  to 
chulc5oclte  generally  does  not  seem  to  be  far  advanced. 

The  relations  of  the  enargite  to  the  other  sulphides 
(except  the  chaloocite)  are  not  definitely  shown*  In  most  oases, 
it  io  oleerly  a  replacement  of  the  carbonate  of  the  wall-rook* 
Small  definite  crystals  have  been  observed,  in  the  unaltered 
limestone,  with  no  traces  of  earlier  or  later  sulphides.  (Fig.  121) 

Certain  enargite  crystals  In  the  11  iestone  show  signs  of 
mechanical  changes,  beln^  bent  and  fractured.  The  fractures 
were  cemented  by  calcite  or  by  oovellite,  of  the  early  typ  . 
Bornite  and  ohaloopyrito  were  observed  in  close  associations 
th  the  oovellite  in  relations  similar  to  those  described  on 
page  256  which  suggest  that  the  enargito  is  an  earlier  product 
than  the  iron-bearing  sulphides. 


. 

• 


Unde.r  the  microscope,  the  polished  surfaces  of  certain 
sections  of  the  enargite  crystals  were  found  to  be  distinctly 
pleocrolo,  when  the  specimen  was  rotated  in  polarized  light. 
When  tho  longer  axis  is  parallel  to  the  plane  of  polarization 
(the  plane  of  tho  reflecting-  mirror),  the  mineral  assumes  a 
pinkish  tint;  at  right  angles  it  becomes  white  or  slightly 
bluish  when  contrasted  with  the  normal  enargite  white*  tii-ny 
sections  however  show  no  trace  of  pleocroisra. 

Chaloopyrite.  Besides  the  ohaloopyrite  previously  de- 
scribed whloh  is  a  replacement  of  bornite  in  the  lattice 
structures,  the  mineral  also  occurs  in  small  grains  which  are 
clearly  contemporaneous  cr  earlier  than  th3  bornite  with 
which  they  ere  associated.  In  tho  typical  cases,  the  two  ages 
of  chalcopyrite  may  be  ro&dily  separated,  but  in  lesa  definite 
forme,  in  whioh  more  complete  replacement  may  have  obscured  the 
original  relations,  it  is  difficult  or  impossible  to  distinguish 
the  . 

Tho  earlier  chalcopyrite  forms  clean  grains,  asroolat 
with  the  bornite  ir.  mutual  structures  for  the  most  part,  but 
occasionally  corroded  by  the  bornito  or  broken  by  its  veinlets 
in  ways  which  convincingly  show  its  earlier  origin,  (figure  126 

The  chaloopyrite,  however,  is  an  uncommon  feature  in  the 
deposits,  and  probably  never  was  of  importance. 

Luaonite.  Voinlets  or  grains  of  a  pinkish  mineral,  which 
has  not  been  positively  identified,  have  been  observed  In  a 
number  of  specimens  iu  the  ohalcocite  or  the  lose  common 
minerals. /  It  agrees  in  tnineralo  graphic  properties  with  the 


265 


Iu2on1te  in  Dr.  LTurdooh's  standard  collection.   It  resembles 
onargite  In  general  properties,  "but  possesses  a  deeper  pink 
color  in  reflected  liftht  and  is  never  pleocroio. 

The  mineral  does  not  e?iet  in  euf-'ioient  ouantiti^s  in  the 
Kanneoott  ores  to  permit  analytical  determination,  "but  qualita- 
tive tests  on  small  fragments  indicate  I/he  presence  of  arsenic 
and  trie  absence  of  antimony  and  bisoiuth. 

The  luzonite,  or  at  least  the  "pink  enargite",  is  a  rather 
late  product, for  its. veins  break  all  other  sulphides  except 
chalcooite.   It  roplaoe;  iiornite  in  the  complex  ohaloopyrite- 
bornite  mixtures,  producing  an  apparent  lattice  of  ohaloopyrite 
in  a  field  of  luaonite.  Elsav/here  it  breaks  the  ohaloopyrite  in 
veinlets,  and  even  cute  boldly  across  the  broad  plates  of 
'.•ovellite.   In  several  cases,  it  I'orrae  borders  around  the 
cryetnls  of  oovellite,  a-  i  "  doporited  at  a  later  time. 

It  plays  e  prominent  part  in  tho  peculiar  aonoentrio 
structures  found  in  various  parts  of  the  ore,  and  preserves  the 
curves  of  these  J -r  tures  even  after  the  less  resistant  minerals 
hnve  boon  oomplotely  replace^  by  chaloooite. 

Tennantite.  Tennantite  is  rarely  abundant  enough  to  "be 
visible  in  the  hand-specimen,  but  it  is  common  in  certain  complex 
mixtures  o  •"  bornito,  ahaloopyrite  and  covellite.  It  is  in  pert 
a  replacement  of  ohaloopyrite,  as  corroded  coren  of  th    :"neral 
are  common  in  the  tennantite  grains.  The  grains,  however, 
usually  possess  oryott-lline  outlines,  and  their  arrangements  in 
raany  cases,  sugffeet  that  they  were  formca  along  the  ./alls  of  open 


spaces. 

The  crystals  are  often  rimmed  \?ith  thin  borders  of  ohal- 
oopyrite  .vifeh  indicates  conclusively  a  recurrence  of  the  for- 
mation of  this  mineral.  Fino  voinlets  of  the  le-tor  bornite 
also  out  the  tennantite  crystals.  The  tennantlte  is  also 
partially  replaced  by  luzonite  which  develops  in  one  or  two 
instances  a  lattice  structure  identical  in  appearance  with 
that  between  bornite  and  later  minerule.  The  tennantite  is 
relatively  resistant  to  rep]  a   :nt  by  CDvellite  and  chaleooite. 

Pyrita.   ryrite  is  very  unoommon  in  the  ore-bodies,  but 
It  is  fairly  wide-spread  as  a  sparse  scattering  of  email  cubes 
in  the  lower  siliceous  bo      the  Chitistona  formation,  where 
it  apparently  ni-s  no  connection  with  the  mineralization  whioh 
produced  the  copper  deposits.  Under  tha  flat  fault  in  the 
Jumbo  ...ino ,  a  fo     11  bunches  ware  found  associated  with 
partially  oxidize  copper  ores  and  Much  limonite.  It  is  possible 
that  the^e  oiay  be  of  the  sort  previously  mentioned,  and  were 
in  the  limestone  before  the  ores  were  formed.  Small  grains  have 
been  observed  in  tha  raidst  of  chaloocite  and  covallite,  but 
th&y  are  of  rare  occurrence. 

Sphalerite.  A  few  large  grains  of  sphalerite  have  been 
observed  in  severe!  specimens  but  they  yield  no  information  con- 
cerning the  relation  oi'  the  mineral  to  tho  other  sulphides. 
The  sphalerite  i.    iier  than  tha  chalcocite,  but  apparently 
not  readily  repl&.oed  by  it. 

Galena.   inly  a  few  small  epeolce  of  galena,  doubtfully 
identified  by  microohemioal  tests,. /ere  observed. 


Concentric  and  "bunded  structures.   Information  concerning 
the  mineral  sequence  presented  on  the  preceding  pages  h&8  "been 
gained  chiefly  from  certain  peculiar  knots  in  tho  mass  of  the 
monotonously  uniform  ohalcocite,  in  which  the  rarer  minerals 
of  the  deposit  are  concentrated.  In  the  majority  of  these 
oases,  the  various  sulphides  posse SB remark able  concentric  or 
"banded  structures,  ./hi oh,  on  account  of  their  genetic  signifi- 
cance are  worthy  of  separate  discussion.  Chaloopyrite  and 
bornite  apparently  form  the  ground  mass,  upon  which  most  of  the 
other  sulphides  have  been  built.  The  formation  of  the  chaloopy- 
rlte  and  bornite  was  succeeded  "by  the  deposition  of  tennantlte, 
both  as  a  partial  replacement  of  the  earlier  sulphides  and  as 
crystals  a  parently  forming  drusy   coatings  on  the  walls  of 
small  vugs.  A  later  phase  of  the  mineralization  again  pro- 
duced ohalcopyrite,  closely  followed  by  covellite  in  abundance 
with  a  little  lusonite(?)  and  bornite.  The  ohaloopyrite  of  the 
second  generation  forms  rims  about  the  tennantite  crystals  or 
cuts  them  in  veins*  It  is  most  abundant,  however,  as  a  replace- 
ment of  Tiornite,  in  complex  aggregates  of  spines  as  previously 
described.  The  covellite  is  always  abundant.  It  is  prominent 
in  large  radial  flakes  in  the  centers  of  the  oonoentic  areas. 
?rom  the  relations  to  the  crystalline  tennantite  bands,  it  is 
very  probable  that  it  constitutes  he  final  filling  of  open 
spaces  remaining  in  the  earlier  minerals,  or  else  a  replacement 
of  other  material  ,;ith  similar  radial  structure  which  had 
filled  euoh  cavities.  There  is,  iuwever,  little  evidence  of 
any  earlier  nineril  associated  with  the  oovelllte  in  this  form. 


'OOaiq  SAJ  ••'.     9S  i«1l 

' 
,  »tiO< 


Inni  ^ 


Elsewhere  the  oovellite  replaoes  bornlte,  and  In  places  so 
thoroughly  that  little  of  the  original  sulphide  remains.  Here 
and  there,  smell  angular  grains  of  chaloopyrlte  or  of  bornite 
ooour  between  the  covelllte  crystals.  Ho  olue  Is  given  as  to 
their  relativ^  a^es.  They  are  probably  contemporaneous,  but 
it  oannot  "be  stated  positively.  A  little  bornite,  In  feeble 
veinlets  is  apparently  formed  at  about  the  same  time  as  the 
crystalline  oovellite.  The  volnlets  out  ohaloopyrite  of  both 
agee,  and  also  the  tennantite.  The  luzonite  is  in  part  oon- 
tamporeneous  and  in  part  later  than  the  crystalline  oovellite. 
It  forms  veins  in  the  tennantite  and  oovellite,  but  also  occurs 
as  sharp  crystals,  euhedral  toward  the  oovellite  fields.  In 
a  few  Instances,  fine  ^ims  of  the  luzonite  were  observed  follow- 
ing the  serrate  outlines  of  the  oovellite  blades.  Chaloocite 
replaoes  all  of  the  preceding  minerals  and  structures.  In  soat 
speoi  iens,  it  ic  present  only  as  veins  or  bands  parallel  to  or 
breaking  across  the  complex  aggregate;  In  others,  more  extreme 
replacement  hao  left  only  coarse  sheaves  of  oovellite 
( figures  12#-129  )  or  rings  or  bands  of  luzonlte  or  of  tennantite. 
Some  trace u  of  these  minerals  are  al'-vays  present  in  the  pebbly 
ohalcoolte,  or  in  the  mammillary  ohaleocite,  and  it  is  very 
probable  that  these  structures  v--er-)  inherited  from  such  complex 
masses  of  onrlier  sulphides. 

Enarglte  is  rare  in  the  knots  of  concentric  sulphides.  It 
is  chiefly  a  direct  replacement  of  the  limestone,  rather  than  an 
open  space  deposit  or  o  replacement  of  other  sulphides. 


atata  i 

• 

.>«?&!  K.' 

• 

!     •1»W     ' 


A*r  Art  ? 


• 


:o   {  ?5  ^eiw>. 


269 


Oxidation.  Distribution.   The  products  of  oxida- 
tion are  prominent  in  all  parts  of  the  deposits.  In  no 
place,  ho\?9Ver,  is  the  alteration  of  the  original  min- 
erals complete,  but,  on  the  other  hand,  there  are  few 
Places  where  no  traces  of  oxidation  exist.  The  degree 
of  oxidation  varies  locally  v>ith  changes  in  the  ore 
minerals  or  wall,  roofcs  but,  in  general,  It  is  surprisingly 
constant  throughout  the  deposit. 

certain  structural  features,  hovever,  off°.r 
especially  favorable  conditions.  Where  cross-breaks 
of  post-mineral  age  intersect  the  veins,  the  alteration 
is  found  to  be  most  Intense.   Bedding  planes  and  the 
fractures  along  which  the  ore  has  been  localized 
-ilMO  afford  channel  ways  for  descending  solutions;   in 
the  Jumbo  mine,  for  example,  the  upper  surface  of  the 
-one  of  the  flat  fault  is  a  particularly  favored  locus 
for  oxidation,  but  it  is  less  marked  in  the  Bonanza 
Mine.   In  general,  the  cross-breaks,  which  allow  un- 
altered surface  solutions  to  descend  to  great  depths 
through  barren  and  chemically  stable  limestone,  are 
more  important  agents  In  promoting  oxidation  than  the 
breaks  parallel  to  the  ore,  for  in  these,  later 
solutions  descending  from  the  surface  are  quickly  robbed 


of  their  powers  of  oxidation  by  the  ertnily  alt  r  I 
smlphides  in  the  upper  parts  of  the  deposit.   Through- 
out the  msnive  sulphide-ore-,  the  alteration  IB  con- 
fined to  the  surfaces  of  joints  or  snail  cracks,  or  to 
the  walls  of  vugs.   The  alteration,  however,  is  feeble; 
it  is  rarely  pervasive,  and  only  where  an  isolated 
chaloocite  stringer  happens  to  be  attacked,  does  it  ap- 
proach completion.  Nevertheless,  the  oxidized  copper 
u.inerale  in  the  aggregate  fora  a  sufficient  fraction 
of  the  ore  to  have  warranted  the  building  of  a  special 
plant  for  their  treatment  by  an  ammonia  leaching  process. 

The  sulphides  are  exposed  on  the  present 
surface,  and  show  no  greater  alteration  there  than  in 
the  deepest  workings.  Broad  faces  of  chalcoclte,  broken 
by  the  mechanical  disintegration  of  the  cliffs,  have 
not  developed  more  than  a  tarnish  fron  their  exposure 
to  the  air.  At  the  Bonanza  Kine,  the  disintegration 
of  the  vein  has  enriched  the  trilus  and  the  araill 
^l^nior  '•     it  to  such  an  extent  that  the  ice  and 
broken  rock  Constitute  a  f'ood  ore.   This  persistence 

of  sulphides  ir.  looae     ,-ial,  exposed  as  favorably 

\ 
as  possible  to\  tr^fe  influence  of  oxidation,  illustrates 

forcibly  the  nefli^ible  extent  of  aurficial  deooiapo- 
i 

f;ition  undor  present,  climatic  conditions. 

X 


oi 


Underground  Temperatures.-  The  mine  temperatures  do 
not  rise  above  the  free sing  point  of  water  at  any  time  of 
year,  except  in  a  few  placea  on  the  Tipper  levels  where  the 
ventilation  is  good,  or  in  confined  drifts  where  many  men 
are  working.  The  mines  are  therefore  absolutely  dry,  even 
on  the  lowest  levels.   Ice  is  a  common  mineral,  occuring 
as  splendid  frost  cryatals  on  the  walls  of  drifts  near  the 
surface,  and  as  partial  or  complete  fillings  of  fissures  or 
other  cavities  at  all  depths.  There  is  no  circulation  of 
water  at  present,  consequently  no  oxidation  is  possible  ex- 
cept by  cold  dry  air,  which  is  negligible. 

Water  Level.-  There  is  no  indication  of  a  frosen 
groundwater  level.   The  continuation  of  abundant  oxidation 
to  the  bottoms  of  the  nines  indicates  that  under  earlier 
conditions,  the  water  level  was  below  the  deepest  workings 
developed  at  present,  which  are  more  than  1000  ft.  below  the 
crest  of  the  ridge  above  the  Bonanza  and  Jumbo  Mines.  This 
deep  oxidation  indicates  almost  certainly  that  the  topog- 
raphy proceeding  the  present  glacial  period  was  one  of  marked 
relief,  and  therefore  of  active  erosion. The  alteration  of 
the  sulphides  was  checker]  by  the  increasing  aridity  due  to 
the  glacial  climate,  and  the  various  phases  of  the  oxidation 
have  been  preserved  v;ith  unusual  completeness* 

Minerals  due  to  Oxidation.  -  The  minerals,  obviously 
formed  by  oxidising  processes,  are  as  follows;  (1)  malachite, 
(2)  limonite,  (3)  covellite,  (4)  antlerite,  (5)  azurite,  (6) 


272 


arsenates  of  copper,  (7)  clialcanthite  and  (8)  cuprite . 
They  are  listed  In  the  probable  order  of  their  abundance. 
Some  quartz  in  the  antlerite  may  be  due  to  oxidation* 

Malachite  is  the  commonest  product  of  oxidation.  It 
is  abundant  as  a  replacement  of  chalcocite,  or  aa  cavity 
fillings  in  vugs  or  other  open  spaces.  As  has  been  de- 
scribed, it  penetrates  the  chalcocite  in  veinlets,  whose 
orientation  is  controlled  by  the  structure  of  the  sulphide, 
producing  lattice  patterns,  or  scalloped  bands  in  the  materi- 
al with  the  concentric  structure.   It  is  also  a  replacement 
of  limestone  to  a  slight  extent.  Coarse  crystals  of  calcite 
with  malachite  developed  along  their  cleavages  were  observed. 
Malachite  produced  in  this  way  is  far  less  abundant  than 
that  which  has  replaced  chalcocite  or  the  other  sulphides 
directly. 

Azurite  is  closely  related  to  the-  malachite  in  origin. 
It  is  unusually  abundant  in  proportion  to  malachite  in  the 
Kennecott  ores.  Vugs  in  the  chalcocite  lined  with  azurite 
crystals  are  very  common,  and  in  nearly  all  places  where 
malachite  is  developed,  there  ia  some  azurite  formed  with 
it.  Ho  definite  age  sequence  between  the  two  carbonates 
could  be  established  however.   In  some  places  one  would  be 
the  older,  in  others  the  reverse  would  be  true.  There  is  a 
slight  tendency  for  the  ratio  of  azurite  to  malachite  to 
increase  in  the  heart  of  massive  ore-bodies,  and  to  decrease 
in  the  zones  of  more  intense  oxidation. 


Limonite  is  widely  distributed  throughout  the  deposits. 
It  is  moat  intensely  developed  in  close  association  with 
the  sulphides.  Stains  penetrate  the  limestone  or  are  dif- 
fused through  gouge  for  notable  distances,  but  the  percentage 
of  iron  is  much  lower  than  in  material  adjacent  to  the  ore. 
Pure  massive  limonite  is  rare  however.   It  ia  nearly  always 
mixed  with  malachite  or  azurite,  or  else  with  residual  limey 
material. 

The  abundance  of  the  limonite  is  surprising  when  the 
low  iron  content  of  the  ore  is  considered.  In  the  largest 
chalcocite  masses,  as  in  the  Jumbo  Mine .where  there  is  al- 
most no  iron,  limonite  is  least  abundant;  in  the  few  places 
where  born ite -ind  chalcopyrite  were  found,  limonitic  altera- 
tion is  very  prominent.  This  relation  suggests  that  the 
limonite  is  derived  directly  from  residual  iron-bearing  sul- 
phides in  the  chalcooite,  but  the  distribution  of  abundant 
limonite  is  by  no  means  limited  to  the  places  where  the 
iron-rich  sulphides  are  present. 

As  has  been  stated,  no  gossan  remains  at  the  present 
surface.  Limonite  is  as  plentiful  in  the  deepest  levels  as 
in  the  shallow  parts  of  the  ore. 

Covellite  is  the  first  product  of  the  oxidation  of 
chalcocite  at  Kennecott.   Its  relation  to  the  chalcocite 
is  conclusively  shown  by  the  field  associations  and  con- 
firmed by  its  distribution  along  malachite  veinlets  ob- 


QG 


a 


ener 


^oolaxfo  erit  at  aeJ5i 
er  iw  eeoxjlg  e  emjwa  on  Trf  i 


served  tinder  the  microscope.   Zonea  of  chalcoclte  partially 
altered  to  oovellite  exist  between  unaltered  chalcocite  on 
one  side  and  oxidized  minerals  on  the  other.  Under  the  mi- 
croscope, rosettes  and  sheaves  of  covellite  commonly  accom- 
pany malachite  veinlets  in  chalcocite  (Fig.  103),  The  spots 
of  covellite,  spreading  out  from  the  channol-ways,  resemble 
the  products  of  decay  advancing  into  wood.  It  is  usually 
closely  followed  by  the  carbonates,  and  there  can  be  little 
doubt  that  they  are  successive  products  of  the  same  general 
processes. 

Antlerite  and  Chalcanthite  are  associated  with  secon- 
dary covellite  in  a  remarkable  ore-body  in  the  Junbo  Mine. 
A  large,  roughly  pipe-shaped  mass  of  oxidized  ore  occurs 
parallel  to  the  dip  of  the  beidlng  at  base  of  one  of  the 
large  sulphide  stopes.  The  overlying  chalcocite  is  the 
pure,  crystalline  type  common  in  the  Jumbo  Lline.  It  passes 
without  sharp  boundary  into  a  purplish  friable  material, 
resembling  covellite,  but  porous  and  with  a  low  specific 
gravity.  In  the  intermediate  zone  between  the  two  extremes, 
chalcanthite  occurs  in  an  intermittant  band  of  1/2  to  1 
inch  in  thickness.  From  the  results  of  chemical  studies  on 

this  material  by  Dr.  2.  2.  Allon  and  Dr.  H.  E.  Merwin  of 

>•. 

the  Geophysical  Labratory,  the  following  information  was 
sent  us:-  Che  composition  of  the  covollitic  material  is:- 


275 


"Cu  S  50.44  per  cent 

Cu  S  04.  5EK0   45.20  ' 

4*56 


100.  £0  per  cent 

The  quartz  crystals  are  all  v;ell  formed  and  each  is  doubly 
terminated.  The  45$6  copper  sulphate  was  a  surprise  as  it  was 
not  detected  under  the  microscope.  Each  grain  of  covellite 
is  very  minute  and  is  probably  surrounded  by  a  zone  of  sul- 
phate". 

The  presence  of  the  quartz  was  not  suspected.  The  over- 
lying chaloocite  contains  practically  none,  consequently  it 
may  have  been  introduced  with  the  oxidizing  sol\itions. 

In  the  field  the  covellite  can  be  detected  cutting  the 
chaleocite  in  a  net-work  of  fine  veinlets.  The  material  is 
very  difficult  to  polish,  but  even  on  imperfect  surfaces,  the 
relation  to  chaleocite  was  clear.  The  covellite  is  without 
question  a  replacement  of  chalcocite. 

That  the  reacting  solutions  contained  copper  sulphate 
is  indicated  by  the  presence  of  the  chalcanthite.  Chalcan- 
thite,  however,  is  very  uncommon  under  normally  humid  condi- 
tions, and  its  formation  here  must  be  attributed  to  the  in- 
creasing aridity  at  the  beginning  of  the  glacial  period. 


1.  S.  T.  Allen,    Private  communication  to  L.  C.  Graton, 
Jan.  1916. 


,no 


a«w 


Q&$ 


Chalcolite 


Transverse      section 
Scale.   :  - 

10    feet 


Ch&lcoc  it  e 


LonqituJinal     section 


37»  Chalcocite-covellite-antlerite  ore-body, 

Jumbo  i.iine. 


The  aone  of  covellitic  material  which  is  a  foot  or  so 
in  width,  forms  a  rim  around  an  underlying  larger  mass  of  light 
greenish  material,  which  'in  the  field  we  believed  was  malachite 
contaminated  with  sulphates.  The  boundary  betv/een  the  covellite 
and  this  oxidized  product  is  fairly  sharp,  but  fine  stringers 
of  the  sulphide  extend  in  lacy  masses  several  x'eet  from  the 


.    s>U. 

I 


.IB. 


line  of  contact.   Specimens  of  the  purest  material  which  we 
collected  were  studied  by  Dr.  B.  E.  Allen,  who  wrote  as  fol- 
lows concerning  the  mineral  :  -   "The  particular  basic  sul- 
phate of  copper  which  you  find  in  Alaska  turns  out  to  be 
3  Cu  0.  303  2H20,  and  is  in  all  probability  identical  with 
antlerite  which  is  in  turn  identical  with  stelznerite.  It 
is  very  difficultly  soluble.   I  am  inclined  to  connect  its 

formation  with  the  presence  of  limestone. Of 

course,  basic  copper  sulphate  is  formed  by  the  action  of 
water  on  a  dilute  copper  sulphate  solution  which  contains  no 
free  acid."   In  later  work  in  the  Geophysical  Laboratory,  it 
was  found  that  the  product  of  the  reaction  of  a  10>  cuprl!c 
sulphate  solution  on  marble  in  the  lump  at  room  temperature 
yielded  a  product  much  like  the  material  from  the  Jumbo  .Mine. 

tfrom  ohis  evidence  it  seems  very  probable  that  the  basic 
sulphate  is  the  product  of  solutionscarrying  copper  sulphate 
reacting  with  the  underlying  limestone,  xhe  field  relations 
are  in  accord  with  this  interpretation.   Ehe  lace  work  of 
covellite  veinlets  may  be  resi'dues  of  chalcocite  veinlets  in 
original  limestone.   Very  similar  conditions  have  been  ob- 
served on  the  edges  of  unaltered  chalcocite  ore-bodies, 

Cuprite  is  very  rare  in  the  deposits  in  the  limestone. 
One  little  nest  of  octahedra  coated  with  malachite  was  found 


1.  Private  communicate  to  Jj.  C.  Graton. 


. 


he: 


et 


in  the  Bonanza  :.!ine,  bxit  the  mineral  was  not  observed  else- 
where except  in  small  amounts  under  the  microscope.  The 
common  oocurrenoe  of  cuprite  associated  with  native  copper, 
described  in  connection  with  the  greenstone  deposits,  offers 
an  interesting  contrast,  and  indicates  the  control  of  wall 
rook  on  the  character  of  the  oxidation.  Native  copper  has 
not  been  observed  in  the  deposits  in  the  limestone. 

Arsenatea  of  copper  are  formed  in  snail  quantities  by 
the  oxidation  of  the  enargite-bearing  ore.   They  are  in  most 
cases  mixed  with  malachite,  and  have  not  been  identified  min- 
eralogically.  Mixtures  of  onar»ite  and  chalcooite  have  been 
observed  to  oxidize  more  readily  than  the  pure  enargite.  The 
chalcocite  is  apparently  the  more  resistant. 

The  influence  of  oxidation  on  the  wall-rock  is  manifested 
chiefly  by  the  replacement  of  calcite  or  of  dolomite  by  copper 
carbonates, basic  sulphates,  or  limonito  respectively.  The 
limestone  being  inert  to  simple  oxidation  shows  no  traces  of 
descending  solutions  away  from  the  ore  except  veins  of  calcita 
or  dolomite  in  coarser  crystals  than  the  grain  of  the  rock. 
However,  near  the  ore,  especially  where  the  development  of  li- 
monite  is  most  intense,  the  limestone  is  altered  to  a  sandy, 
incoherent  material,  apparently  by  the  partial  leaching  of 
the  rock,  which  is  &&£  readily  attributed  to  the  attack  of 
the  acid,  produced  by  oxidation  of  the  sulphide  or  the  hydro- 
lization  of  the  ferric  sulphate  which  produced  the  limonite. 


M 


'.  Xw 

. 
I 

Off*    t 
&J.P.&- 

. 

leetsoo  aJt 

,  eio  erf*  i«er.    .        .>we 


279 


This  is  especially  notioable  in  the  Jumbo  Mine  near  where 
pyrite  was  found  and  on  the  under  side  of  sulphide  masses. 

THE  ORIGIH  OP  THE  COPPER  DEPOSITS  IU  THE  GREEHSTOHE. 

Similarity  t£  Other  Regions*  -  The  mineralization  in 
the  greenstone  is  similar  in  many  important  features  to 
that  described  in  numerous  widely  separated  regions  where 
copper  minerals  occur  in  basic  lavas.   The  forms  assumed 
(veins  of  rather  slight  persistence,  disseminations,  and 
amygdule  fillings)  the  metallic  minerals  (bornite,  chalco- 
pyrite,  native  copper  and  cuprite)  and  the  gangue  minerals 
(calcite,  quartz,  epidote,  chlorite  and  -  although  not 
abundant  -  zeolites,  datolite  and  prehenite)  are  all  charac- 
teristic of  this  world-wide  type  of  mineralization, 

Source  of  the  Metal .  -  The  distribution  of  the  copper 
through  all  parts  of  the  Copper  River  region  where  the  green- 
stones occur,  supported  by  the  common  association  of  copper 
with  basic  extrusives  elsewhere,  points  definitely  to  the 

lavas  themselves  as  the  source  of  the  metal.  Lindgren 

g 
writes:   "Basic  igneous  rocks  such  as  gabbro,  diabase,  basalt, 


1.  Lake  Superior  region;  White  River  District,  Alaska; 
eastern  Oregon  (20  miles  west  of  Baker  City);  Triassic  Traps 
of  Hew  Jersey  and  Conn.;  near  Lurray,  Va. ;  South  lit.,  Pa.;  the 
Bay  of  Pundy,  Hova  Scotia,  the  Paeroer,  north  of  Scotland; 
Sterling  in  Scotland;  Oborstein  a.d.  Nabp,  Germany;  Sao  Paulo, 
Brazil;  the  Kristiania  Region,  Horway;  Hew  Guinea;  the  Trans- 
balkalian  provinces  on  the  Dochida  River;  Monte  Catini,  near 
Livorno,  Italy.   (Por  a  general  discussion  and  references  to 
literature  of  the  above  districts  see  Lindgren 's  Mineral  De- 
posits, p.  392.) 

2.  Mineral  Deposits  (Ma  Graw-Hill,  Hew  York,  1913)  p.  393, 


., 

ij 

. 

i:  .Becfl 
oo  aJLfncertiffi  leqqoo 

. 

• 

'.?A  ie  vit«j 

biqe    ,          . 

• 


• 

at    ,TJ  - 

,  o.l*& 

. 


.  : 


2SO 


some  andesites  and  basaltic  flows  designated  melaphyres 
or  amygdaloids  probably  always  contain  copper,  in  some 
cases  as  much  as  0*1  or  0.2  per  cent.,  but  commonly  about 
0,02  per  cent,  of  the  metal.  According  to  Volney  Lewis 
and  F.  P.  Grout,  the  copper  is  present  as  a  silicate  pos- 
sibly in  part  as  a  sulphide  such  as  bornite  or  chalcooite-" 
In  the  Kenneoott  district,  ohaloopyrite  has  been  observed 
as  an  accessory  mineral  in  the  lavas  apparently  remote 
from  notable  mineralization,  and  in  all  probability  the 
sulphide  is  a  primary  constituent  of  the  rook.  Ho  analy- 
ses of  the  greenstone  have  been  made  however,  but  the  gen- 
eral distribution  of  copper  is  shown  by  the  commonness  of 
faint  stains  of  malachite  in  fractures  along  which  waters 
have  percolated, 

Theories  of  Cone ent rat i on  of  the  Copper  in  Basic  Lavas. 
Although  most  writers  are  in  general  agreement  in  regard- 
ing the  basaltic  lavas  as  the  source  of  the  copper,  there 
is  considerable  difference  of  opinion  concerning  the  mechan- 
ism by  which  the  concentration  of  the  metallic  mineral  was 
effected,  Whitney,  Pumpelly  and  Wadsworth  have  attributed 
similar  ores  in  the  Lake  Superior  region  to  concentration 
and  deposition  by  descending  waters,  Van  Hise  regards  the 

1,  Moffit  and  Cappa,  Loo,  cit. 


')h  8* 


.i«o  -i-fco   S»Q   -o  1*0  6u 

• 

••A     ».f  9il^  lo    •d'.noo  tst' 

.')C« 

. 
^rl'c   ,• 

aoor-  .cr£  a.v. 

og      ,  tsncoo  ^xt^Jtiq  e 


3ieif«v, 


.d 

.^s^txrife 
ni  a0ic 


. 


, 


281 


zeolites,  which  are  usually  closely  associated  with 
the  copper  minerals,  to  be  products  of  descending  sur- 
face waters  or  similar  percolating  waters  in  the  zone 
of  cementation,  the  material  for  the  zeolitization  hav- 
ing been  extracted  from  the  rock  itself  ,  although  he 

somewhat  later  attributes  the  Lake  Superior  copper  de- 

2 

posits  to  ascending  solutions*  Weed  believes  ores  as- 
sociated with  basic  lavas  in  the  Appalachian  region  in 
Virginia  and  Pennsylvania  to  have  been  formed  by  infil- 
trations from  the  surface.  According  to  a  theory  ad- 

at 

vanoed  by  Lane  for  the  I*ake  Superior  deposits,  the  lavaa 
are  believed  to  be  submarine  effusions,  and  the  concen- 
tration of  the  copper  is  attributed  to  the  action  of 
heated  sea  water  on  the  beds.  Zeolitic  deposits  in  the 

Watchung  basalts,  in  which  a  little  copper  occurs,  are 

4 
believed  by  C.  H.  Fenner  to  be  the  result  of  lava  flows 

encountering  water  in  shallow  lakes  or  playaa.  The  chan- 
ges effected  are  attributed  to  readjustments  of  the  miner- 
al combinations  along  fractures  and  in  the  rook  itself 


1.   C.  H.  Van  Hise,  Monograph  47,  U.S. G.S., 1904,  pp 
333  and  633. 

2»  W.  H.  Weed,  Types  of  copper  deposits  in  the  southern 
United  States  -  T.A.I.H.S.,  Vol.  30,  1900,  pp.  449  -  504. 

3.  A.  C.  Lane,  Salt  »?ater  in  the  Lake  mines,  Proc., 
Lake  Superior  Min.  Inst,  vol.  12,  1906. 

4*  C*  U.  Fenner,  The  Watohung  basalt  and  the  parageno- 
8i3  of  its  zeolites,  Annals.,  H.  Y.  AC  id.  Sci.t  vol.  20, 
part  2,  pp.  97  -  187,  1910. 


•noo  la 


under  the  influence  of  steam  and  hot  waters*  Lewi!  as- 
sumes hot  solutions  of  cuprous  sulphate  released  from  in- 
trusive trap  magma  to  account  for  native  copper  in  certain 
other  How  Jersey  deposits.  In  the  region  about  the  head- 
waters of  the  White  Kiver,  Alaska,  chalcocite,  native  cop- 
per and  a  black  hydrocarbon  occur  with  oaloite  and  various 
zeolites  in  amygdaloids,  possibly  of  the  same  age  as  the 

Nikolai  greenstone*  The  deposits  are  described  by  Adolph 

g 
Knopf  ,  and  are  attributed  to  hot  solutions  circulating 

through  the  lavas  soon  after  their  extrusion*  The  flows 
are  believed  to  be  submarine  from  fossil  evidence  in  in- 
terbedded  tuffs  and  breccias* 

She  development  of  albite,  epidote,  chlorite  and  da- 
tolite  in  certain  of  the  veins  in  the  Hikolai  greenstone 
indicates  that  they  are  not  the  product  of  cold  meteoric 
waters,  for  these  minerals  are  not  formed  under  such  con- 
ditions* The  later  minerals,  calcite  and  the  zeolites, 
are  undoubtedly  the  products  of  milder  conditions,  but 
probably  part  of  the  same  sequence*  Veins  similar  to 
those  at  the  Brie  Mine,  which  break  across  the  greenstone- 
limestone  contact  can  not  be  related  to  cooling  effects  of 
the  lavas, and  were  without  question  formed  under  later  and 
very  different  conditions.  The  shorter  veins,  throughout 
the  formation,  which  rarely  break  through  more  than  three 


1*   J.  Y.  Lewis,  Gaol.  Sur*  Hew  Jersey,  1906,  p.  131, 
£•  A.  Knopf;  Loo*  cit. 


. 


• 


•zabass 


o  L&fygoR&  ai 
J01>  9i{'I     ,eco*aneei, 

, 

08    61  Vf  C    Affi 


«_   ixx    .^«V    «n»    ^«    w^.*..,  ^ 


or  four  flows,  may  be  the  products  of  the  final  stages 
of  the  vulcanism.  but  they  are  quite  similar  to  the  later 
veins.  '  The  information  concerning  the  submarine  or  sub- 
aerial  extrusion  of  the  lava  is  too  indefinite  to  base 
any  theories  upon.  Ho  evidence  favoring  a  submarine  ori- 
gin was  observed  near  Kennecott;  the  liraonitio  horizons 
suggest  land  surfaces,  but  do  not  prove  it* 

The  intrusions  of  the  porphyries  in  post- Jurassic 
time  -,vere  wide-spread  phenomena,  and  the  unequal  heating 
to  which  the  country  must  have  been  subjected,  undoubted- 
ly accelerated  the  circulation  of  the  ground-water.  Heated 
solutions,  traversing  the  copper-bearing  rocks, are  agents 
capable  of  causing  noteworthy  changes  and  may  be  the  cause 
for  some  of  the  greenstone  veins,  such  as  those  breaking 
the  limestone-greenstone  contact.  The  importance  of  this 
circulation  and  possible  structural  relations  will  be  dis- 
cussed more  fully  on  the  pages  dealing  with  the  origin  of 
the  ores  in  the  limestone. 

The  long-continued  volcanic  activity  in  the  Tertiary 
may  have  had  a  similar  effect,  but  it  was  probably  neither 
as  localized  nor  as  intense  as  the  heating  caused  by  the 
porphyry. 

The  ores  on  Uugget  Creek  in  the  XUskulana  District 
are  stated  to  be  later  than  important  structural  changes 
in  the  greenstone.   if  so,  they  are  not  products  of  the 
cooling  lavas,  but  must  be  attributed  to  later  causes. 

1.  A.  Wandtke,  Oral  communication. 


at? 


u 


.0 


i 


Porphyry  intrusions  are  common  In  this  re- 
gion also,  and  may  be  the  indirect  agents,  as  dis- 
cussed above*  Certain  ores  in  this  district  are  close- 
ly related  to  the  porphyry,  and  are  probably  contact 
effects,  but  their  mineral  character  (pyrite,  magne- 
tite and  ohalcopyrite  with  garnet)  is  in  striking 
contrast  to  the  ores  in  the  greenstone  in  general  and 
argues  strongly  against  the  porphyry  as  the  general 
source  of  the  copper.   The  porphyry  intrusions  may 
have  had  an  important  part  in  the  formation  of  the 
deposits,  but  it  is  not  believed  that  they  supplied 
the  metal* 

The  chaloooite  in  the  greenstone  deposits 
is  clearly  secondary  in  part,  and  a  replacement  of 
bornite.   Hear  Kennecott,  chalcocite  is  only  slight- 
ly developed  in  the  greenstone*   In  the  Xuskulana 
district, the  ohaloocite  is  of  two  ages*   The  older, 
which  is  more  abundant,  is  either  a  primary  inter- 
growth  or  a  replacement  of  bornite.  If  it  is  of 
the  latter  origin,  it  may  possibly  be  secondary 
and  related  to  an  older  topography.  The  ohaloocite 
of  later  age  is  due  to  feeble  enrichment  from  a  sur- 


«d 


:.5ni  9;  mn 


J&ti 


OT^  »/i^  aJt  e^ioc. 
i  A  baa    ,  ,-n»4 


face  of  rapid  degradation,  probably  of  inter-glacial 


Re'surae'.   The  source  of  the  copper  in 
these  deposits  is  believed  to  be  the  lavas  them- 
selves;  the  concentration  of  the  metal  may  have  tafcen 
place  in  part  by  heated  solution"  as  the  lavas  cooled, 
but  it  ia  probable  that  the  stronger  veins  are  of 
1-ator  origin,  and  were  deposited  by  the  circulation 
of  solutions  heated  and  set  in  motion  by  the  in- 
trusions of  the  post-Jurassic  porphyry.   The  chal- 
cooite  is  secondary  in  part,  and  a  replacement  of 
bornite.   Chalcooite  in  graphic  and  siiailar  struct- 
ures in  bornite  from  the  Kuskulana  District  is  poss- 
ibly primary;   the  origin  of  chalcocite  in  these 
structures  will  be  considered  separately  in  a  later 
section  of  the  paper. 


2X6 


THE  ORIGIN  OF  THE  COPPER  DEPOSITS  IN  THE  LIMESTONE. 

Summary  of  Significant  Features  From  our  observation 
in  the  field  and  in  the  laboratory,  and  from  the  results  of 
vrork,  in  the  OeophyRical  Laboratory,  many  features  concerning 
the  ores  lu  th<  limestone  have  been  definitely  established. 
The  moat  important  points  which  must  be  taken  into  consider- 
ation in  any  theory  of  origin  for  the  deposits,   are   sum- 
marized in  a  following  paragraph. 

^ho»  first  ten  observations  bear  chiefly  on  the 
problems  concerned  with  the  primary  origin  of  the  deposits  - 
the  source  of  the  copper,  the  manner  of  concentration,  and  its 
^resent  loci.  The  question  whether  the  chalcocite  was  'formed 
directly  as  a  replacement  of  limestone  or  a  replacement  of 
older  sulphides  depends  largely  on  the  observations  listed 
f rom  (  11 )  through  (17),  while  the  problem  of  the  secondary 
or  primary  origin  of  the  chalcocite  requires  the  consideration 

of  the  remaining  points  of  the  list  as  well. 

The  list  of  a ignifioant  features  is  as  follows:  — 

(1)  The  dependence  of  the  ore  on  the  greenstone-limestone 

contact  (  the  Mother  Lode  deposit  an  apparent  exception  ); 

(^)  the  widespread  distr lout  ion  of  copper  in  the  under- 
lying greenstone; 

(3)  the  dependence  of  the  ore  on  definite  fractures,  and 
the  iacfc  of  disseminated  sulphides; 

(4-)  the  definite  lower  limit  (  stratigraphically )  of  the 
ore  in  the  larger  mines; 


;     -       .  i    *. 


:  aeTj-tffiel  j;  _e  '10   Jell 


(5)  the  gradual  decrease  In  ore  (ohalooclte)  passing 
upward,  strati^raphioally; 

(  6 )  the  sulphides  a  replacement  of  limestone  In  nearly 
all  places  where  the  relation  Is  shown; 

(  7 )  the  occurrence  of  small  Knots  In  the  ore  character- 
ized by  the  complex  concentration  of  the  less  abundant  min- 
erals, and  usually  by  concentric  structures ; 

(£)  the  laefc  of  alteration  in  the  wall  rocl:  (except  par- 
tial recrystallization); 

(  9 )  the  occurrence  of  enargite  in  notable  amounts  in 
the  Bonanza  L'ine; 

(10)  the  lacfc  of  definite  relations  between  the  ore 
and  any  intrusive  rocfc; 

(11)  the  great  size  of  the  ore-bodies; 

(  12)  the  variation  In  texture  of  the  ohalcocite  (  steely 
and  crystalline  types); 

(  13 )  the  dominance  of  chalcoclte  over  other  ore-minerals 
and  its  unusual  purity; 

(1U-)  the  results  of  analytic  and  synthetic  vorfc  on  Xen- 
neoott  ohalcooite  by  the  Geophysical  Laboratory;  constantly 
low  copper  content  and  specific  gravity  of  the  oh  loocite; 
chaloocite-oovellite  solid  solutions;  the  high  and  low  tem- 
perature forms  of  chalcocite; 

(  15 )  the  replacement  relations  which  the  chalcocite 
exhibits  toward  all  other  sulphides  (except  a  part  of  the 
covellite  ); 


sac 

ilOUSTSJlF  '1C 

. 

•o  s cacti 


'^^^^^^^^^^^^^B^^^H 


' 
' 
• 

. 

;( 8«{H;>f  :^io 

*o  c  {  ^s.  } 

' 


iscieof 


2X8 


(16)  the  common  distribution  of  traces  of  bornite 
throughout  the  chaleocite  (  most  abundantly  in  the  steely 
Bonanza  glance,  less  abundantly  in  the  crystalline  Jumbo 
glance ); 

(17)  the  definite  structural  relations  occasionally 
shown  between  the  chalcocite  and  bornite; 

(Iff)  complete  lacfc  of  surfioial  alteration  under  pres- 
ent climatic  conditions; 

(19)  wide-spread  but  partial  oxidation  throughout  the 
mines;   laci  of  dependence  or.  present  surfaoe,  and  lack  of 
-Ilialnution  with  depth  within  the  limltw  of  the  mines  in  1916; 

(20)  the  existence  of  physiographic  conditions  which 
afford  land  surfaces  of  possible  relation  to  the  secondary  pro- 
cesses,- pre-glacial  topography  of  notable  relief;   older 
Tertiary  topography  of  gentle  relief;  pre-voloanlo  peneplain 
in  early  Tertiary,  and  pre-Kennecott  post-Chltlstone  peneplain; 

(21)  the  existence  of  structural  features  -  low  dip  and 
flattening  of  limb  of  anticline  to  the  southwest;   the  in- 
fluence of  the  flat  fault  as  a  barrier  for  descending  solutions; 

(  d2  )  the  slightly  greater  abundance  of  bornite  in  the 
chaleocite  from  the  Erie  Mine,  at  a  lower  elevation  on  the 
limb  of  the  anticline  than  the  Bonanza  and  Jumbo  deposits; 

(23)  the  occurrence  of  bornite,  in  slightly  more  abundant 
ai.iounts,  in  the  open-out  at  the  Bonanza  iiine  under  the  zone 
of  the  "flat- fault." 


.fittl 
ill    ' 


ifa  eJin. 


^  rs  ^r  ^.  ?- 

jfiwo^  XO     1O  A_L 

oe   er;  o   ser.    . 

' 

;nlfllvi. 


2S9 


The  Origin  of  the  Primary  Ore.  The  only  possible 
sources  of  the  copper  as  far  as  our  knowledge  of  the  district 
extends  ar<3  the  greenstones  and  the  post-Jurassic  porphyries. 
The  remoteness  of  the  Tertiary  volcanics,  and  *he  lack,  of 

a;.y  known  mineralization  associated  with  them  raaKes  it  ex- 
trer.ely  Improbable  that  the  copper  was  derived  fron  any  phase 

of  their  activity. 

The  close  association  between  porphyry  intrusions 
and  ores  in  many  of  the  copper  camps  in  the  Southwest  suggests 
at  ths  outset  that  similar  relations  may  exist  in  this  dis- 
trict. The  porphyry,  however,  near  Kennecott  shows  no  traces 
of  mineralization,  either  in  the  heart  of  the  Porphyry  Mt. 
stocfc  or  near  the  edges,  or  in  the  smaller  bodies.   At  the 
Erie  Mine,  a  three-foot  dike  cuts  ths  vein,  but  it  is  uniainer- 
alized  and  aost  probably  later  than  the  ore,  although  this 
cannot  be  established  with  certainty.   On  the  southwestern 
side  of  Porphyry  lit.,  low-grade  gold  ores  have  been  found. 
Little  information  was  available  and  the  region  was  riot 
visited.  The  gold  is  associated  with  pyrite,  anrt  the  miner- 
alization is  apparently  very  different  from  that  near  Kenneoott. 
In  the  Kuskulana  region,  a  few  deposits  of  little  importance 
seem  related  to  porphyry  contacts;  as  has  been  mentioned  the 
.•:ineralizatlon  Is  in  distinct  contrast  to  that  in  the  green- 
stone or  in  the  limestone,  and  indicates  a  different  origin 
from  the  larger  deposits  of  the  region. 


air 


-1- 


290 


There  is  the  possibility  that  the  ores  were 
deposited  from  solutions  which  had  emanated  from  the  por- 
phyry magraa,  but  hich  had  travelled  ;?orae  distance  before 
depositing  their  load.  The  presence  of  enargite  in  the  ore 
suggests  an  igneous  Bouree,  but  the  confinement  of  the  ores 
to  definite  fractures  and  the  lacfc  of  Bilicificatlon  both 
ii.'iieate  that  the  conditions  of  ore-format  ion  were  not  in- 
tense.  Nevertheless  it  is  difficult  to  believe  that  emanations* 
evenfrona  remote  magma,  would  be  so  poor  in  silica  and  so 
relatively  rich  in  the  metala  to  have  produced  ore-bodies  of 
the  Kennecott  ty.     At  3isbse,  however,  rich  ores  do  occur 
in  almost  unaltered  limestone  in  parts  of  the  mineralized  zone 
remote  from  the  prophyry,  yet  there  is  little  doubt  that  they 
wer"e  derived  from  the  igneous  rocfc.  Consequently  the  poss- 
ibility that  the  Kennecott  ores  were  formed  in  this  way  can  not 
be  completely  eliminated,  although  the  probability  that  it  is  the 
correct  view  is  not  great. 

The  deposits  in  the  limestone  are  believed  by  iloffit 

1  2 

and  Capps  and  by  J.  ].'.  Irving  to  have  been  derived  from  the 


.  lely  disseminated  copper  in  the  greenstone.   It  ie  suggested 
that  the  leaching  of  the  limestone  may  have  been  facilitated 
by  the  heating  due  to  the  intrusions.   "It  is  believed  that 
the  copper  ta&en  into  solution  by  circulati.    xtsr  was  carried 
ii.to  trunk  channels  and  depot?  ited  there  when  the  conditions 
favorable  .....  Most  frequently  deposition  tooK  place 


1.  Loo.  cit.,  (Bull.  445),  pp. 

2.  Loc.  nit. 


noil 


in  the  greenstone  formation,  but  at  times  the  copper-bearing 

waters  passed  outside  the  greenstone  and  into  the  overlying 

.  1 
limestone  before  giving  up  their  mineral  load." 

The  greenstone  affords  a  competent  source  for 

the  copper  frou  a  quantitative  standpoint,  for  it  is  copper- 
bearing  almost  wherever  exposed.  Concentration  of  copper  in 

the  greenstone  is  known  to  have  taken  place  in  pont-Chitistone 
time,  for  ve inlets  of  the  typical  greenstone  typ^s  break  across 
the  greenstone-limestone  contact  at  the  Erie  Mine  and  else- 
where.  The  relation  of  the  ores  to  the  contact  is  striking 
in  all  cases  except  in  the  Mother  Lode  Mine.  Even  there  it 
is vnot  impossible  that  the  ore  may  increase  or  at  least 
continue  until  the  contact  is  encountered.   Small  stringers 
of  the  chaisocite  in  the  limestone  are  not  infrequently  en- 
countered as  one  follows  the  contact  along  the  cliffs  between 
the  Jumbo  and  Erie  Mines.   Definite  fissuring  is  lacking  in 
all  these  cases,  however,  and  none  of  the  seams  persist  many 
feet,  but  usually  die  out  along  bedding  planes  or  short  cross- 
fractures.   The  lower  siliceous  limestone  beds  are  apparently 
unfavorable  for  mineralization.  At  the  Jumbo  and  the  Bonanza 
Mines  the  existence  of  strong  and  persistent  fissures,  both 
of  the  vertical  zone  and  the  flat  fault,  gavs  the  ascending 
solution^  acoen,  to  the  purer  limestone,  and  also  formed  im- 
portant channel  ways  in  which  a  concentration  could  take  place, 

1.  Mofflt  and  Capps,  LOG.  oit.,  p.  S2. 


. 

'  •          • 


ne$wJ> 


i  :»I«Jt 


292 


It  remains  a  disputed  fact  among  geologists  whether 
a  metallic  concentration  of  this  sort  can  be  effected  by 
col^l  meteoric 'waters.  However,  ordinary  meteoric  waters 
undoubtedly  become  more  active  agsnte  if  accelerated  both  in 
'•ir'culation  and  chemical  action  by  dli'ferentlal  heating  due 
to  igneous  intrusions.   In  the  Kennecott  district,  the  wide- 
spread intrusions  of  post-Jurassic  porphyry  probably  had  this 
effect.   Where  definite  circulation  along  channel  ways  was 

established,  copper  leached  from  the  greenstone  would  be  pre- 
cipitated, if  conditions  were  favorable.  Where  these  channel- 
ways  crossed  the  contact,  ind  the  fissures  per-  isted  in  the 
limestone,  or  encountered  other  dominant  fractures,  these  con- 
ditions were  supplied;v/here  the  limestone  is  unbrfcfcen,  the 
mineralization  rarsxy  extends  far  above  the  greenstone. 

The  reactions  involved  in  the  solution  and  precip- 
itation of  the  copper  according  to  this  theory  of  origin  are 

not  known.   It  is  possible  that  the  copper  was  transported 

1 

in  a  colloidal  state.  Tolman  and  ClarJc  have  shown  thai  col- 
loidal dispersions  of  copper  sulphides  may  be  induced  by 
hydrogen  sulphide,  and  flocculated  by  calcium  carbonate.   The 
existence  of  hydrogen  sulphide  in  sufficient  quantities  in 
the  greenstone,  however,  is  a  mere  speculation.   It  or  other 
agents  could  conceivably  hive  been  provided  by  emanations  from 
the  porphyry.  The  precipitation  of  the  colloidil  copper  by 
calcium  carbonate  solutions  offers  an  attractive  explanation 
for  the  localization  of  the  ores  in  the  lower  beds  of  the 


1.   The  dispernion  and  precipitation  of  copper  sulphides 
from  colloidal  suspension,  Econ.  Geol.,Vwl.  ^,  p.  559-592, 


limestone.   The  hypothesis  finds  possible  support  in  the 
Concentric  and  mannilary  structures  observed  here  and  there 
in  the  ore,  which  resemble  certain  colloidal  products,  but 
£rom  our  present  knowledge  of  colloids  it  is  doubtful  whether 

the  great  bodies  of  sulphides,  clearly  replacements  of  lime- 
stone could  have  been  formed  in  this  way. 

The  solutions  deposited  little  except  the  metals, 
sulphur  and  arsenic.  The  quartz  ind.  silicates  in  the  veins 
in  the  greenstone  probably  were  derived  from  readjustments 
of  mater ill  in  the  adjacent  wall  rocfc,  with  little  trans- 
portation or  addition  involved,  for  these  minerals  cease  ab- 
ruptly at  the  limestone  contact.   It  IR  possible,  however,  that 
the  two  rocfcs  exert  sufficiently  different  precipitation  ef- 
fects on  the  solutions  to  account  for  the  .changes. 

in  resume.,  the  theory  of  derivation  of  the  copper 
by  leaching  of  the  greenstone  offers  a  competent  source  for 
the  metal  and  a  satisfactory  mechanism  for  its  concentration, 
an  explanation  of  the  relation  of  the  ores  to  the  greenstone 
contact,  and  of  the  lac*  of  sillcification  In  the  limestone. 
Its  laiin  points  are  supported  by  positive  evidence.   Opposed 
to  it  may  be  mentioned  the  lack  of  strong  veins  in  the  green- 
atone  beneath  the  larger  ore-bodies,  and  the  occurrence  of 
enargite  in  the  limestone.  The  first  objection  is  not  vital; 
the  material  v/as  not  necessarily  derived  from  the  greenstone 
immediately  belov:  the  Contact,  but  may  have  migrated  to  the 


-'I; 


favorable  ohannelways  In  the  limestone  from  notable  distances. 
The  occurrence  of  enargite  is  puzzling.   At  Butte  it  is  re- 
garded as  a  product  of  intense  mineralization;  hero  the  condi- 
tions ware  undoubtedly  rcl;  tively  mild.   So  enargite  was  ob- 
served in  the  greenstone,  and  although  the  presence  of  small 
quantities  of  arsenic  throughout  the  lavas  is  not  impossible, 
its  presence  is  a  serious  difficulty  in  the  way  of  complete  ac- 
ceptance of  the  leaching  theory. 

THE  NATU3K  OF  THE  CHALCOCITB. 

Three  possible  explanations  for  the  Kennecott  chalcooite 
may  be  offered:  (1)  diroct  deposition  from  heated  ascending 
solutions,  as  a  direct  replacement  of  the  limestone,  (2)  a 
replacement  of  bornite  and  to  a  lesser  extent  of  other  sulphides 
by  asceruling  solutions,  or  (3)  a  replacement  of  bornite  and  to 
a  lesser  extent  of  other  sulphides  by  descending  meteoric  solu- 
tions.  It  must  be  admitted  at  the  outset  that  the  evidonoe  is 
conflicting.   In  the  final  sumrnary  the  weight  given  the  various 
arguments  will  probably  throw  tha  scales  one  way  or  the  other, 
but  due  to  the  different  values  given  the  evidence  by  several 
investigators  interested  in  the  problem,  a  close  agreement  is 
hardly  to  be  expected. 

Evidence  from  studies  in  the  Geophysical  Laboratory.  In 
the  course  of  work  at  the  Geophysical  Laboratory  knowledge  of 
oortain  properties  of  ohalcocite  was  obtained  which  has  a  dis- 
tinct bearing  on  the  Kenneoott  problem.  Synthetic  copper  sul- 
1  _  E.  Posnjak,  K.T.Allen,  unu  H.E.Mer»in,   Loo.oit. 


: 

' 

. 

I 

;TT  Aaf  3el*T«arc' 


295 


phidas  formed  under  certain  conditions  were  found  to  contain 
more  sulphur  then  IB  demanded  by  the  ratio  2  Cu  :  3.  It  was 
shown  that  these  products  were  solid  solutions  of  cuprous 
and  ouprio  sulphide.  Pure  cuprous  sulphide  -  chaloooite  -  was 
found  to  "be  dimorphous,  the  inversion  temperature  being  at  about 
91°C..  Increasing  the  amount  of  cupric  sulphide  dissolved  in  the 
cuprous  sulphide  raises  the  inversion  temperature.  This  takes 
place  until  cuprio  sulphide  reaches  a  concentration  of  about  8 
per  cent,  after  which  an  inversion  is  no  longer  observed.  The 
crystals  of  chaloooite  formed  at  higher  temperatures  in  various 
ways  are  isometric.   tfhen  ooolecl  below  91°  C,  inversion  takes 
place,  and  if  etched  the  characteristic  etch-cleavage  of  or- 
thorhombio  chaloocite  develops.   Cuprous  sulphide,  however, 
containing  more  than  &  per  cent  of  ouprio  sulphide  if  formed 
above  91°  C  does  not  develop  the  characteristic  etch-cleavage 
of  orthorhombio  ohalcocite  at  lower  temperatures, but  retains  an 
octahedral  parting  which  etches  out  more  or  less  clearly. 

Tlu   octahedral  parting  of  the  artificial  material  yields 
patterns  on  tine  polished  surface  which  are  similar  to  the  etch- 
structures  of  the  Kenneoott  chalcooite  (Pig.l39)«  "It  is  fur- 
ther significant  that  analyzed  samples  of  the  Bonanza  ore  show 
about  nine  per  cent  of  dissolved  cuprio  sulphide,  a  quantity 
sufficient  to  prevent  isometric  crystals  which  may  have  baen 
formed  above  91°  from  going  over  into  the  orthorhornbio  form  as 
the  cooling  progressed.   Etched  surfaces  of  some  of  the  ana- 
lyzed samples  of  this  ore  contain  grains  in  irregular  veins 


;«r 


eifrcw  georitf 


which  etoli  like  ohalcooite  (ortho  rhombic)  and  are  thus  shown 
to  be  the  low  temperature  for..  •   They  may  have  formed  above 
91°  and  contain  too  little  cuprio  sulphide  to  prevent  their  in- 
version, or  they  may  have  formed  below  the  temperature  of  in- 
version*''^ In  examining  material  sent  to  the  Geophysical  Labor- 
atory from  our  collections,  H.  K.  Ller.via  found  a  number  of 
twinned  crystals  in  the  chalcooiie,  which  he  believes  from  oaftas- 
uremente  of  the  angles  to  be  octahedral  twine  of  ohaloocite.  If 
his  interpretation  is  correct,  it  is  the  first  recognition  of 
isometric  ohalcocite  in  nature* 

The  possibility  of  ohalcocite,  with  over  8  per  cent  ouprio 
sulphide,  when  formed  ut  ordinary  temperatures,  assuming  the  oc- 
tahedral form,  hay  not  been  positively  eliminated.  It  is  stated, 
however,  in  their  paper  thet  if  solid  solutions  containing  over 
0  per  cent  of  covellite  oouid  be  crystallized  at  temperatures 
just  below  91°  ,  they  would  probably  etch  like  ordinary  ohalcooita. 

Front  th»  work  of  the  (Jeo  physical  Laboratory  a  fairly  strong 
case  is  presented  for  the  thesis  that  the  Kenaecott  chaloooite 
is  of  primary  origin  and  formed  at  a  temperature  above  91°  G, 
but  as  it  will  be  shown  later,  the  octahedral  structure  of  the 
chalcocito  may  be  explained  in  another  way  that  does  not  demand 
a  high  temperature  for  Its  formation,  which  consequently  weakens 
the  foroe  of  thoir  conclusions. 


1  -  Kugene  Poenjak,  B.T.Allen,  and  H.il 
£  -  F.  ,T«Allen,  Private  communication. 


297 


£videnoe  that  ths  chaloooite  IB  a  replacement  of  bornite. 
Structures  identical  with  the  isometric  patterns  described 
above  have  been  observed,  however,  in  ohaloooite  whioh  is 
clearly  a  replacement  of  bornite.  They  were  first  described 
by  Graton  and  :.iurdoeh^in  1913.  The  etch-cleavage  in  some  of 
the  ohaloooite  was  believed  by  them  to  be  inherited  from  born- 
ite. It  was  definitely  pointed  out  that  such  cleavage  in  sec- 
ondary ohaloooite  could  not  in  all  cases  be  distinguished  from 
the  regular  (isometrio)  cleavage  vto  ioh  they  believed  to  be 
characteristic  of  primary  ohalcooite. 

In  a  more  recent  article  on  the  etoh -patterns  of  ohalco- 
oite,  C.  ?.  Tolrnan,  Jr.,  comes  to  the  same  conclusion  regard- 
ing the  replacement  origin  of  the  chaloooite  with  the  triangu- 
lar or  rectangular  pattern,  and  in  addition  states  positively 
that  this  pattern  proves  the  ohaloooite  to  be  a  replacement 
of  an  earlier  mineral,  generally  of  bornite.  Material  from 
the  Bonanza  iiine  at  Kenneoott,  and  from  the  Apache  Mines, 
Hma  Co.,  Arizona,  is  described  by  him  in  whioh  the  derivation 
of  ohaloocite  from  bornite  is  clearly  shown.  The  blue  and 
whita  patterns  on  slightly  tarnished  polished  surfaces  are  re- 
garded by  Tolman  as  certain  proof  of  replacement  origin.  It 
is  believed  "that  the  color  is  due  to  the  unmixing  of  the  sol- 
id solution  rof  ohaloocite  and  oovellitel  whioh  develops  sub- 

1  -  L.C.Graton  and  J. Murdoch,  Loo.olt. 

2  -  Loo.oit. 


&%  ox* 


. 


. 


. 

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microscopic  crystals  of  oovellite.   White  ohaloooite  of  later 
age  is  described,  whioh  is  "believed  to  be  due  to  supargene  ac- 
tion, preceding  the  development  of  oovellite  and  malachite. 
Tolman  summarizes  the  parageneeis  of  the  ore  minerals  of  the 
Bonanza  deposit  as  follows: -"First  group  of  minerals; -Bo rnite, 
klaprothite  klaprotholite;,  and  galena;  temperature  of  formation 
probably  relatively  high  for  copper  deposits,  as  Prof. Rogers 
has  found  klaprothite,  bornite  and  ohaloocite  to  be  grouped  to- 
gether in  most  of  the  ohalcocite  deposits  thought  to  have  been 
formed  by  ascending  solutions,  and  at  temperatures  above  those 
at  which  downward  enrichment  takes  place.  Second  group  cf  min- 
erals; -Blue  and  white  ohaloooite  of  the  first  generation,  sec- 
ondary after  bornite.  The  temperatures  governing,  and  the 
source  of  the  solution  causing  this  alteration,  are  unknown. 
Third  group  of  minerals.-  This  includes  a  set  of  minerals 
formed  progressively  under  conditions  of  increasing  oxidation 

and  represented  as  follows:  Second  generation  of  white  ohaloo- 

(oovellite  and) 
oite-j  (chaloopyrite  )  — »  tenorite  (and  cuprite )_>  malachite 

— »  azurite.  The  group  is  formed  at  about  0°C.,  as  much  of  the 
ore  is  frozen  throughout  the  year.  It  is  the  i*esult  of  the 
present  v^dose  circuit; tion,  and  tho  copper  is  migrating  chiefly 
as  malachite . " 

The  mineral  sequence  given  by  Tolman  is  confirmed,  in  a  gen- 
eral way,  by  our  work;  but  from  field  observations  and  larger 
collections  our  mineral  list  is  considerably  greater.  Ho  klap- 
rotholite  was  observed,  however,  and  the  rarity  of  galena  hardly 


•»'. 


. 


entitles  it  to  "be  mentioned  with  bornite.  Tolman'e  statement 
of  the  temperature  of  formation  of  the  primary  ore  is  "based  on 
little  real  evidence.  In  his  third  -rroup  of  minerals,  we  oan 
not  confirm  tho  occurrenoe  of  tenorite.  Limonito  is  not  men- 
tioned, although  it  is  an  extremely  abundant  mineral.  It  is 
doubtful  if  asurite  can  in  gener&l  be  regarded  as  a  replacement 
of  malachite .   Copper  migrating  as  malachite  by  means  of  a 
frozen  vadose  circulation  is  rather  surprising. 

To  return  more  definitely  to  the  topic  under  discussion, 
thero  is  good  evidence  that  an  important  part  of  the  chaloooite 
at  Kenneoott  is  a  replacement  of  bornite.  The  fine  grains  of 
bornite,  common  in  many  parts  of  the  ore,  are  most  simply  in- 
terpreted as  residues.  When  they  are  of  sufficient  size,  re- 
placement relations  are  usually  shown.  The  triangular  pattern 
in  tho  ohaloooite  is  similar  to  the  structure  remaining  after 
a  lattice  replaoeiaont  of  bornite.  The  distribution  of  bornite 
specks  and  spines  in  a  few  specimens  lends  force  to  this  inter- 
pretation for  the  origin  of  part  of  the  Kennecott  ohaloooite. 

If  the  ohaloooito  is  a  replacement  of  bornite  two  origins 
are  possible;  (1)  it  is  the  final  product  of  the  ascending  pri- 
mary solutions,  or  (2)  it  is  a  product  of  descending  meteoric 
solutions. 

Secondary  versus  Primary  orirln  for  the  Chaloooite .   It  is 
obvious  thf.t  secondary  enrichment  is  virtually  Impossible  under 
the  present  climatic  conditions  at  Xennecott.  It  is  also  un- 
likely thr.t  enrichment  of  any  importance  was  associated  with 


' 


•frag 


300 


the  oxidation  now  prominent  throughout  the  deposit.  Tho  ox- 
idation is  not  intensive  and  is  limited  to  fractures  for  the 
most  part  upon  which  the  ohaloocite  shows  no  dependence.  The 
topography  and  climate  immediately  preceding  the  present  gla- 
oial  epoch  probably  supplied  the  necessary  conditions  for  its 
development.   From  the  evidence  presented  by  Gapps,  two  gla- 
oiul  periods  seem  probable;  the  oxidation  is  possibly  the  pro- 
duct of  the  inter-glacial  period. 

The  various  surf GOO s  to  whioh  secondary  ores  could  be  re- 
latoJ  have  boon  described  in  detail  on  pages  22.J-231*-.  The  most 
recent  is  the  surface  of  low  relief  developed  during  the  period 
of  pre-glaoial  erosion,  which  to  Judge  from  remnants  could  not 
have  been  far  above  the  mines.  During  this  time  the  ores  may 
well  have  boon  exposed  to  long  continued  oxidizing  influences 
in  so  permeable  a  medium  as  the  Chitistone  limestone.  The  long 
period  of  erosion  which  produced  the  even  surface  beneath  the 
Tertiary  lavas  could  also  havo  afforded  favorable  conditions  for 
enrichment.   If  projected,  this  surface  ?;ould  pass  not  far  above 
the  top  of  Bonanza  Peak.  The  peneplain  out  across  the  Chitistone 
limestone  and  the  greenstone,  whioh  serves  as  a  floor  for  the 
Jurassic  sediments,  may  likewise  have  offered  a  surface  from 
whioh  enrichment  could  take  place  if  the  ores  were  then  in  ex- 
istence. At  a  point  less  than  two  miles  from  the  mines,  the 
erosion  at  this  time  was  sufficient  to  strip  the  limestones  ooia- 

1  -  S>.  3.  Cupps,    oo.cit. 


'iMD  Jt     £ 


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, 


., 


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pletely  from  the  greenstone.  Although  it  is  not  possible  to 
determine  the  actual  relation  of  these  surfaces  with  greater 
precision,  it  is  evident  that  there  were  at  various  periods 
physiographic  conditions  whioh  may  have  permitted  enrichment 
on  an  important  scale. 

Structural  conditions  at  the  Jumbo,  Bonanza  and  Erie 
:.iines  are  such  that  a  email  amount  of  erosion  would  make  a 
large  amount  of  copper  available  for  enriching  solutions.  The 
plunge  of  The  ore -bodies  is  approximately  parallel  to  the  dip 
of  the  limestone.  There  is  good  evidence  that  the  dip  flat- 


West 


East 

Poss/i/e    position    Of 

early    Tertiary   surface 


3#  •  Sketch  illustrating  relation  of  ore 
to  successive  erosion  surfaces. 

tens  to  the  southwest  as  the  crest  of  the  anticline  is  ap- 
proached. Consequently  the  ores  which  have  been  eroded  prob- 
ably had  a  lower  dip  than  those  remaining.  If  a  30°  dip  is 
assumed,  which  is  the  least  favorable  case,  it  is  evident  that 


. 


.01308    tux*' 

T  O  » 

al   err 


302 


if  the  surface  is  lowered  100  feet,  200  feet  of  the  ore  meas- 
ured along  the  dip  would  be  removed.   With  a  decreasing  dip 
this  ratio  would  increase.  The  concentration  of  the  descending 
solutions  laterally  upon  the  underlying  ores  would  be  effi- 
ciently effected  by  the  barrier  offered  by  the  gouge  layer  of 
the  flat-fault.  These  factors,  in  addition  to  the  well-known 
ease  with  which  oxidation  penetrates  to  great  depth  in  limestone, 
offer  unusually  favorable  conditions  for  the  ooncentration  of 
copper  by  descending  solutions. 

The  occurrence  of  a  slightly  greater  amount  of  bornite  In 
some  of  the  chalcocite  from  the  Erie  Mine  than  la  comraonly  found 
in  the  Xennecott  ores,  is  a  slight  argument  favoring  a  secondary 
origin,  for  tho  Erie  Mine  is  farther  down  the  limb  of  the  anti- 
cline and  its  ores  would  have  been  farther  below  the  older  sur- 
faces to  which  the  enrichment  must  have  been  related  than  is  the 
case  in  the  other  deposits  of  the  district. 

In  the  summer  of  1916,  some  chalcooite  with  unusually  abun- 
dant bornite  was  encountered  in  the  open-out  of  the  Bonansa  Mine, 
at  a  horizon  below  the  zone  of  the  flat-fault.   The  bornite  IB 
in  material  which  is  undoubtedly  more  resistant  to  percolating 
solutions  than  the  overlying  rook,  and  its  preservation  in  such 
places  is  quite  in  agreement  with  the  relations  expected  under 
the  conditions  of  the  hypothesis  of  secondary  origin  for  the 
chaloooite. 

Ihe  ease  of  bornite  enrichment  is  another  factor  worthy  of 


~i 


• 
el     '        I  •v 


" 

>on»   ••  j'v.  eJimod  trr-a'- 


. 


303 


no to  in  favor  of  a  secondary  origin  for  the  chaloocite.  The 
presence  of  chaloooite  of  unquestioned  secondary  origin  in  the 
bomite  nodule  from  the  limestone  near  the  Brie  iline  shows  that 
the  enrichment  oan  take  place  in  the  presence  of  calcium  carbon- 
ate, although  the  alteration  in  this  oase  is  on  a  small  scale. 

Opposed  to  the  secondary  prigin  of  the  chaloooite  are  the 
following  arguments: 

The  chalcooite  occurs  in  limestone  which  for  the  most  part 
shows  no  traces  of  aoid  attack.  Although  the  enrichment  of 
bornite  produces  comparatively  little  aoid,  the  great  quantities 
involved  if  all  the  ohalcooite  were  formed  in  this  way  would 
surely  seem  to  have  been  sufficient  to  have  had  a  pronounced  ef- 
fect on  the  wall  rock.  It  should  be  pointed  out  in  this  connec- 
tion, however,  that  according  to  the  equation  established  by 

1  to 

Sies  ;,  Allen  and  Merwin,  bornite  may  alter  a  mixture  of  ohaloo- 

A 

cite  and  oovellite,  as  follows: 

CU5  Pe  S4  +  Cu  S04  =  2  CU£  3  +  2Cu  3  +  F«  304 
with  the  production  of  no  sulphuric  aoid.  The  analyses  of  ap- 
parently homogeneous  chalcooite  from  the  Kennecott  deposits  are 
uniformly  low  in  copper,  and  are  interpreted  by  Dr.  alien  to  in- 
dicate about  9  per  oent  of  dissolved  cupric  sulphide.  In  addi- 
tion there  is  a  small  amount  of  free  covellite  distributed 
rather  generally  through  the  ore'.  This  total »  however,  does  not 
yield  the  ratio  demanded  by  the  equation,  but  it  is  sufficient 
to  reduce  the  amount  of  aoid  produced  by  approximately  a  quarter. 
1  -  Loo.  cit.,p.  461. 


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304- 


The  only  evidence  of  acid  attack  Is  the  sandy  limestone,  which 
was  observed  in  a  few  places  near  the  ore,  some  of  which  was 
associated  with  abundant  limonite,  and  certain  peculiar  irregu- 
lar masses  of  gouge-like  material  of  uncertain  origin*  The 
former  may  be  attributed  as  well  to  the  acid  released  by  the 
hydrolyzing  of  the  ferric  sulphate.  To  what  extent  the  greater 
insolubility  of  the  dolomite  would  influence  its  behavior  toward 
acid  attack  is  unknown. 

If  the  chalcocite  is  attributed  to  the  action  of  descending 
waters,  it  implies  the  movement  of  larger  quantities  of  copper 
sulphate  solution  through  tho  ore,  and  necessarily  through  the 
adjacent  limestone.   It  has  been  established  from  both  chemical 
and  geological  data  that  carbonates  or  basic  sulphates  of  copper 
tend  to  form  under  such  conditions,  which  would  remove  copper 
from  solution  and  hinder  the  formation  of  secondary  sulphides. 
It  is  evident  that  this  process  would  probably  be  of  little  in-* 
fluenoe  on  solutions  descending  through  the  rich  sulphide  ores, 
but  it  would  be  expected  to  check  the  development  of  chalcooite 
in  fine  stringers  in  unaltered  limestone.  The  occurrence  of 
chalcocite  in  thin  veinlets  in  oalcite  or  dolomite,  a  few  of 
which  show  no  traces  of  malachite,  azurite  or  sulphates,  is  a 
strong  argument  against  the  formation  of  the  ohaloocite  from  de- 
scending copper  sulphate  solutions. 

The  replacement  of  such  large  masses  of  bornite  would  in- 
volve the  removal  of  a  great  quantity  of  iron,  probably  as  fer- 


. 
•ajp   »3fil-e7 


>  ofr  Jtatoeqxe  r 

o«  wi 


305 


rous  sulphate.  In  neutral  solutions,  suoh  as  those  in  a  car- 
bonate rook,  limonite  would  form  readily  if  mild  oxidizing  con- 
ditions were  encountered;  if  the  iron  remained  in  the  ferrous 
condition  reaction  with  the  wall-rock  might  be  expected  to  yield 
siderite.  Important  masses  of  siderite  believed  to  be  of  simi- 
lar origin  occur  at  Bisbee.  The  limonite  in  the  deposits  is 
not  as  abundant  aa  would  be  expected  if  the  ohaloocite  were  en- 

* 

tirely  a  replacement  of  bornite  under  secondary  conditions.  The 
distribution  of  limonite  in  part  suggests  that  it  is  a  product 
of  later  oxidizing  conditions  and  not  of  those  which  produced 
the  chaloocite.  Siderite  has  not  been  observed,  and  it  is  cer- 
tainly not  present  in  any  noteworthy  amount  on  the  levels  now 
exposed.  These  observations  offer  definite  objections  to  any 
genetic  process  which  would  involve  the  transportation  of  large 
quantities  of  iron  through  the  limestone  in  the  form  of  soluble 
sulphates. 

Passing  upward  perpendicularly  to  the  stratification,  the 
chaloocite  in  the  veins  gradually  pinches  out*  The  fissures 
continue  and  are  usually  marked  by  reorystallized  limestone 
along  their  walls.  The  chaloocite  in  the  upper  beds  cannot  be 
said  to  be  more  intensely  oxidized  than  that  in  the  lower,  and 
there  is  no  overlying  zone  of  oxidized  ore.  It  is  improbable* 
therefore,  that  the  ohaleocit^  coul...     have  been  formed  by  ver- 
tically descending  waters,  for  there  are  no  known  higher  sources 
from  which  the  copper  could  have  been  derived.  If  seoondary,  the 
copper  along  the  upper  edge  of  the  ore-bodies  must  have  been  de- 


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III 


307 


rived  by  almost  lateral  movement.  With  a  30°  dip  it  is  con- 
ceivable that  descending  solutions  following  the  horizon  of  the 
flat-fault  could  have  spread  far  enough  to  have  accomplished 
this  change  (Figure  35      )  ,  but  it  does  not  aeem  probable 
that  it  could  1  •..,  ,j   en  done  with  such  searching  completeness. 
On  the  whole,  the  continuation  of  pure  oh  aloo  cite  in  small 
stringers  into  the  upper  beds  is  a  strong  argument  against  the 
hypothesis  of  secondary  origin. 

The  arguments  supporting  the  various  hypotheses  of  the  ori- 
gin of  the  ohaloooite  are  summarized  in  tabular  form  on  page  306  . 
It  is  apparent  that  it  is  difficult  to  settle  the  question  defi- 
nitely, but  the  evidence  on  the  whole  presents  a  stronger  case 
for  the  view  that  the  chalcocite  is  primary  than  that  it  is  sec- 
ondary. The  arguments  supporting  the  latter  merely  show  that 
conditions  favorable  for  deep  enrichment  exist;  they  do  not  meet 
the  detailed  objections  raised  against  it.  It  is  certain  that 
part  of  the  chalcocite  at  Kenneoott  is  a  replacement  of  bornite; 
the  poasibility  that  some  of  the  chalcocite  may  be  a  direct  re- 
placement of  limestone  at  temperatures  above  91°  c»,  however, 
cannot  be  eliminated. 

dUUQIAEY  ATC>  CONCLUSIOflS. 


The  mineral  deposits  at  Kennecott  consist  of  two  distinct 

types:  (1)  small  but  numerous  bornite  and  chalcopyrito  deposits 
of  little  commercial  importance  in  the  Nikolai  greenstone,  and 

larrre  ore-bodies  of  ohaloocite  in  the  Chitistone  limestone. 


•Jft 


:o 


*,r 


tne 


f  atcio  t>9 L 


30S 


The  basic  lavas,  now  greenstones,  aro  believed  to  have  been 
the  source  of  the  copper  in  both  types.  In  the  deposits  in  the 
greenstones  themselves,  the  copper  may  have  been  concentrated  in 
part  by  volcanic  after-effects,  but  the  larger  veins  were  pro- 
duced by  later  circulation  of  heated  solutions.  It  is  suggested 
that  the  solutions  may  have  been  derived  from  meteoric  waters, 
accelerated  in  circulation  and  chemical  action  by  the  heat  of  the 
extensive  post -Jurassic  intrusions. 

The  ores  in  the  limestone  are  also  believed  to  have  been 
formed  by  similar  solutions,  which  had  derived  their  copper  con- 
tent from  the  greenstone.  Where  fractures  extended  into  the  lime- 
stone,and  other  structural  conditions  were  favorable,  the  solu- 
tions broke  across  the  contact  from  the  greenstone,  and  deposited 
the  primary  sulphides  of  the  great  ore-bodiea. 

The  primary  sulphides  in  the  limestone  wore  formed  chiefly 
as  replacements  along  the  fractures,  and  never  occur  disseminated 
through  the  rock.  Locally,  deposits  were  made  in  small  open  spaces. 
The  conditions  of  temperature  and  pressure  were  probably  mild*  The 
only  rock- alt oration  is  a  partial  recrystallization  of  the  car- 
bonates of  the  limestone.  No  silicates  and  a  very  subordinate 
amount  of  quartz  occur  in  the  ores. 

The  list  of  ore-minerals  and  a  diagram  of  sequence  are  given 
on  page  So?  •  Bornite,  enargite  and  part  of  the  chaloopyrite  were 
the  earliest  ore-minerals  to  form.  The  bornite  probably  continued 
to.  be  deposited  somewhat  longer  than  the  chalcopyrite,  but  the  two 
minerals  are  believed  to  have  been  essentially  contemporaneous. 


. 


. 


}«£««MC   $Q   f 

srad 


- 


/oe  9  A 


5  *ijl 


uX. 


310 

lo  Ifo  u/    O&  ae.   3  /O 


O&  ae. 

copper  occur,  but  are  not  common.  A  small  amount  of  chalcocite, 
for  example  that  in  the  bornite  nodule  near  the  Erie  Mine,  may 
be  attributed  to  the  period  in  which  the  malachite  a.nd  limonite 
were  developed,  but  it  is  almost  negligible  in  quantity.  The 
oxidation  is  not  related  to  the  present  surface  or  to  the  pres- 
ent climate;  it  took  place  under  pre-glacial,  or  possibly  inter*. 
glacial,  conditions,  when  the  climate  was  milder,  and  the  relief 
somewhat  like  that  at  present. 

If  the  mineralization  continues  with  depth,  there  is  little 
possibility  that  there  will  be  notable  decrease  in  the  richness 
of  the  copper  minerals.  If  the  chaloocite  is  primary,  little 
change  would  be  expected;  if  secondary,  it  can  be  a  replacement 
only  of  bornite,  hence  the  change  to  primary  ore  would  be  a  grad- 
ual one  and  not  marked  by  serious  diminution  in  copper  values. 


Diagram  of  Mineral  Sequence. 


Mineral 

Period  of  Primary 
Mineralization 

Period  of  oxidation 

Tertiary 

Interglaoial 

mite 
Chalcopvrlte 

Enargite 
Pyrlte 

Tennantlte 

Covelllte 
(  primary) 

Luzonite 
Sphalerite 

-  aa 

—  — 

•—  ^™^^^«—  - 

(if  cfafcocitf 



? 

—  '— 

Chalcocite 

Primary  theory 
Secondary  " 

Covellite 
(  Secondary  ) 



Linonite 



Malachite 



Chalcanthite 



Antlerite 



Areen^tes  of 
Copper 

Cuprite 



• 


310 


Bornlte  was  probably  by  far  the  moat  abundant.  The  ohalcopyrite 
is  replaced  to  a  slight  extent  by  tennantite,  which  in  addition 
to  this  relation,  also  oocura  as  fine  crystals  on  the  walls  of 
open-spaces.  The  tennantite  was  followed  by  the  development  of 
oovellite  in  coarse  crystals  in  veins  or  in  radial  masses  prob- 
ably filling  cavities,  riflth  the  covellite,  &  small  amount  of 
luzonite  (?)  was  formed,  as  thin  veinlets,  or  crusts  on  earlier 
crystals*  ,.n  almost  negligible  amount  of  sphalerite  and  galena 
accompanied  the  primary  mineralization. 

The  earlier  minerals  and  their  relations  are  almost  oblit- 
erated by  intense  replacement  by  chalcooite.  The  bornite  was 
most  completely  replaced,  but  all  of  the  earlier  sulphides  suf- 
fered. The  chaloooite  may  be  also  a  direct  replacement  of  the 
wall-rook  in  part,  although  it  cannot  be  settled  with  absolute 
certainty,  it  is  believed  that  the  chalcooite  is  the  last  prod- 
uct of  the  primary  mineralization,  and  not  the  result  of  proc- 
esses of  secondary  enrichment. 

Chaloopyrite  and  a  little  bornite  reoccur,  probably  as  by- 
products of  the  changes  which  produced  first  the  primary  covel- 
lite, and  later,  the  chaloooite,  from  the  earlier  iron-bearing 
sulphides.  If  the  ohaloocite  is  secondary,  part  of  the  ohalco- 
pyrite is  also  secondary. 

The  first  product  of  the  oxidation  of  chalcocite  at  Konne- 
oott  is  covellite.  This  is  usually  followed  by  limonite,  mala- 
chite and  aaurite,  but  in  one  important  case,  by  chalcarthite  ani 
antlerite  (a  basic  sulphate  of  copper).  Cuprite  and  arsenatos  of 


I  a- 


o  aesee 


PART     IV 


GENE  RAL     DISCUSSION 


313. 


PART  IV 

GENERAL  DISCUSSION 

GENERAL  FEATURES  OF  BORHITE  DEPOSITION 
Bornite  in  Deposits  of  MaKiratio  or  Pneumatolytio  Origin 

1 
Some  Properties  of  Magtnatio  Sulphide  Ores.    Mag- 

matio  sulphide  ores  are  commonly  regarded  aa  those  in  whioh 
the  sulphide  minerals  were  forced  directly  from  a  magma  in 
a  manner  analogous  to  the  crystallization  of  the  rook  min- 
erals. The  ores  should  possess  the  structure  of  the  igneous 
rook  with  whioh  they  are  associated,  and  form  an  integral 
part  of  its  body.  The  gangue -minerals  of  such  deposits 
should  be  the  characteristic  minerals  of  igneous  rocks. 
Alteration  products  such  as  garnet,  wollastonite  or  tremolite, 
and  especially  those  of  hydrothermal  origin  suoh  as  epidote, 
serioite,  chlorite,  carbonates  or  zeolites  should  be  either 
absent  or  of  later  origin  than  the  sulphides. 

Sulphides  in  many  pegmatite  dikes  usually  possess 
the  properties  mentioned  above  as  essential  characteristics 
of  Biagmatio  deposits.   In  most  of  these  oases,  however,  there 
is  reason  to  believe  that  the  concentration  of  volitale  con- 
stituents was  greater  than  in  a  normal  magma  arid  that  the 
deposition  of  the  sulphides  was  related  to  these  agents. 
Consequently  for  the  sake  of  completeness  in  classification, 

1.  Compare  Lindgren,  Mineral  Deposits,  p.  735. 


1 


•diJieaoi'I  etaoS 


8i«i«>nlEB  ai 


'-0 


-n  •«l 


313. 

it  seer;  a  proper  to  regard  the  origin  of  such  deposits  as 
pneumatolytio  rather  than  directly  magmatio.  The  two  groups, 
however,  are  closely  associated,  and  no  sharp  line  can  be 
drawn  between  their,,  for  it  is  probable  that  pneumatolytio 
action  plays  a  more  or  less  important  part  even  in  those 
bornite  ores  which  are  moat  closely  related  to  magmatio 
conditions.  With  changes  toward  milder  pressures  and  tem- 
peratures, a  similar  gradation  may  exist  between  pneumato- 
lytic  a-.l  hyirotherrcal  ores,  but  the  point  at  which  minerals 
of  generally  recognized  hydrothermal  origin  appear  may  be 
established  as  the  point  at  which  true  pneumatolytio  condi- 
tions commence  to  give  way  to  hydrotheriral  conditions. 

gyrrhotite-Chalcopyrite -Bornite  Ores.  The  ores 
near  Ookiep,  Little  Namaqualand,  are  the  only  rrag&atio  depos- 
its of  the  pyrrhotite-ohalcopyrite  type,  in  which  bornite 
is  an  important  mineral.  The  distribution  of  the  ore-bodies 
in  broad  lenses  or  sohliers  in  unuaual  baeio  rooks,  which 
exhibit  a  remarkable  degree  of  differentiation,  suggests 
that  the  sulphides  were  derived  directly  from  the  magma. 
The  sequence  of  the  sulphides  and  their  relations  to  the 
rook  silicates  indicate,  however,  that  metasooatic  replace- 
ment also  played  an  important  role  in  their  formation. 

1 

From  the  experinents  by  Vogt,  it  is  very  prob- 


1.    L.  J.   H.    L.   Vogt,    Die   Silikatsohnrelzlosungen,    pt.    1, 
Videnskabe-Selskabets.      Skrifter,    Math.-Naturv.   Klasse, 
Kristiania,    1903,    No.    8.     W.   Lindgren,   Mineral  Deposits, 
p.    761. 


314. 


able  that  sulphides  dissolved  in  a  dry  silicate  malt  would 
be  the  earliest  constituent  to  separate  out  from  &  cooling 
solution.  The  late  position  of  the  sulphides  in  the  se- 
quence of  gangue  arid  ore -minerals  is  clearly  out  of  harmony 
with  these  results,  and  indicates  with  certainty  that  differ- 
ent influences  controlled  the  deposition  of  the  sulphides 
in  nature  from  those  provided  by  the  conditions  of  the 
experiments.  Vogt  suggests  that  the  retention  of  mineral - 
izers  in  the  magma  due  to  the  great  pressures,  was  prob- 
ably the  chief  factor  which  determined  the  late  position 
of  the  sulphides  in  nagrcatio  orea.  Concentration  of  metallic 
constituents  is  effected  in  residual  portions  of  crystalliz- 
ing ir.agaa,  as  is  shown  convincingly  by  ores  in  pegmatites 
and  in  high-temperafture  veins.  In  abnormal  magmas  in  which 
a  high  metallic  concentration  had  been  produced  by  differ- 
entiation, this  final  segregation  would  probably  produce  a 
still  richer  and  more  fluid  extract  capable  of  migration 
and  localization  in  definite  ore-shoots.  Such  late  magmatio 
solutions  would  be  expected  to  corrode  and  replace  the  ear- 
lier silicates,  and, as  the  character  of  the  solutions  changed, 
the  earlier  sulphides  would  become  unstable  and  would  be 
forced  to  yield  to  later  products. 

Such  an  explanation,  which  is  offered  for  the 
Ookiep  ores,  may  also  harmonize  the  apparently  conflicting 
field  and  microscopic  evidence  at  Sudbury.  The  high  metal- 
lic concentration  of  the  ir&gma  in  the  lower  portions  of  the 


.3as«; 


315. 

Sudbury  basin  may  be  attributed  to  direct  differentiation 
in  the  liquid  phase  controlled  by  gravity,  while  the  con- 
centration of  the  sulphiies  into  definite  ore-shoots,  with 
the  accompanying  replacement  of  the  silicates  and  the  suc- 
cessive or e -minerals,  may  be  attributed  to  the  agency  of 
mineralizers  in  the  final  rr.agcr.atio  extracts.   The  importance 
of  niineralizers  in  the  magma  is  suggested  by  the  presence 

of  abundant  znioropegrnatite  throughout  the  overlying  norite 

1 
and  granite.   Tolman  and  Rogers   believe  that  the  Sudbury 

ores  are  late  aagmatic  products  and  claim  to  have  recon- 
ciled the  diametrically  opposed  viewe  of  origin.  They  state, 
however,  that  the  ores  were  not  formed  by  the  sinking  of  the 
sulphide  constituents,  but  offer  no  explanation  for  the  con- 
centration of  the  ores  near  the  bottom  of  the  ir.agwa  chamber. 

2 

Lindgren   states  that  the  temperature  of  forma- 
tion of  magmatio  deposits  proper  ranges  from  700°-  1500°  C. 


He  places  the  temperature  of  the  niagmatio  ores  associated 
with  pegmatite*  at  about  575°C.  Tolman  and  Rogers,  however, 
emphasizing  the  replacement  relations  between  the  high 
temperature  silicates  and  the  sulphides,  state  that  the 
formation  of  sulphides  in  all  types  of  high -temperature  de- 
posits takes  place  probably  not  higher  than  300°-  400°C. 
While  the  value  of  the  work  on  temperature  variations  is 


1.  Loo.  oit. 

8.  Mineral  Deposits,  p.  188. 

3.  Loc.  cit. 


. 


316. 

appreciated,  it  nay  be  undesirable  to  try  to  exprees  with 
too  great  exactness  the  temperature  at  which  the  sulphides 
in  deposits  of  different  types  font.  It  seems  reasonable 
to  assume,  however,  tnat  in  deposits  intimately  related  to 
igneous  activity,  there  is  a  general  decline  in  temperature 
from  the  initial  conditions  of  the  melt  to  the  close  of  the 
mineralization.  Though  local  or  temporary  reversals  in 
this  progressive  cooling  may  be  expeotad,  the  general  tend- 
ency is  probably  sufficiently  dominant  to  justify  the  as- 
sumption that  the  time  relations  of  any  mineral  to  the  other 
products  of  the  mineralization  may  be  regarded  as  an  approxi- 
mate indication  of  the  relative  intensity  of  the  temperature 
under  which  it  was  formed.  In  the  deposits  at  Ookiep,  where 
bornite  is  involved,  the  sulphides  formed  later  than  the 
rock-silicates  (hypersthene,  plagioola.se,  biotite),  and  con- 
sequently their  temperature  of  formation  may  be  assumed  to  be 
lower  than  that  at  which  the  silicates  crystallized.  The 
upper  limit  of  the  temperature  of  formation  depends  on  the 
temperature  of  the  crystallization  of  the  rook  minerals  and 
is  probably  within  the  range  set  by  Lindgren.  There  is  no 
evidence  which  limits  the  temperatures  in  this  case  to  the 
low  values  given  by  Tolman  and  Rogers,  although  the  lateness 
of  bornite  in  the  sequence  of  silicates  and  ore-minerals 
suggests  that  it  was  for  rued  at  a  considerably  lower  temper- 
ature thai;  the  latest  rock-minerals. 

Pueug-.atolvtic  Ores.   In  the  remaining  deposits 


'T£««ezgc 

«1    TOfl 


/i3«Isi   t»d^ 

&  •. 


317. 


described  in  this  group,  the  orea  are  of  a  somewhat  different 
type,  and  clearly  more  dependent  on  pneumatolytio  conditions. 
The  sulphides  occur  in  pegmatites  or  in  other  dike  rooks  in 
which  numerous  miarolitic  cavities  and  the  character  of  the 
associated  minerals  clearly  indicate  the  importance  of  gaseous 
agents. 

In  the  deposits  at  Copper  lit.,  La  Fleur  lit.,  and 
at  Engels,  sulphides  occur  apparently  as  primary  constituents 
of  pegmatite  dikes.  In  the  first  two  deposits,  bornite  is 
the  chief  mineral  in  this  association;  at  Engels  chaloopyrite 
is  more  abundant  in  the  pegmatites.  At  Evergreen  the  bornite 
is  confined  in  a  large  measure  to  the  dike  in  which  the 
parent  rock  occurs,  and  this  is  also  the  case  at  La  Fleur  Mt. 
except  where  the  syenite  and  syenite- pegmatite  dikes  out  gabbro. 
At  Copper  Mt.  and  at  Engels,  however,  the  disseminated 
sulphides  in  the  plutonio  rook  constitute  the  chief  ore-bodies. 

In  the  pegmatites  and  in  the  parent  dikes  at  Ever- 
green and  La  Fleur  Mt.,  the  sulphides  are  later  than  the 
silicates  but  there  is  almost  no  rook  alteration  connected 
with  the  mineralization,  except  changes  attributable  to 
pneumatolytic  agencies,  as  the  development  of  aegirite-augite 
and  fluorite  at  Evergreen,  or  the  tourmaline  at  Engels  or 
Copper  Mt.  The  ores  in  the  plutonic  rook  at  Engels,  Copper 
Mt.,  and  La  Fleur  Mt.,  on  the  other  hand  are  usually  accom- 
panied by  important  changes,  and  are  believed  to  have  been 
deposited  under  a  range  of  conditions,  decreasing  from  the 


- 

-,;$«qefc .  •ioar;xl¥«-     • 

•I    »2lL  '.     10000    t&fc., 

9C 

.; 

aeaUJb 

. 
' 

, 
•  oic    .  8Yffl  -fe 

^5. 
• 

. 
i.jrfj    :.  ;38X 

• 
' 

. 

. 

,  a  f.  eg  • 

• 


318. 


Intensity  of  the  pne  meat oly tic  stages  through  successively 
milder  temperatures  to  the  feebleness  of  final  hydrothermal 
emanations.  The  formation  of  the  sulphides  in  the  wall  rook 
was  preceded  by  the  development  of  hornblende  over  a  wide 
zone  at  Engels,  converting  the  original  norite  to  the  pecu- 
liar diorite  or  metanorite,  but  at  Copper  Mt.  and  La  Fleur 
Mt.  this  change  was  more  closely  confined  to  seama.   The 
sulphides  soon  followed,  but  were  accompanied  by  an  increas- 
ing amount  of  epidote,  serioite  and  chlorite,  indicating 
that  the  original  pneumatolytio  conditions  had  given  way  to 
hydrothermal.  In  the  closing  stages  at  Engels,  the  final 
effects  are  shown  by  the  development  of  carbonates  and  zeo- 
lites. The  association  of  the  hydrothermal  minerals  with 
the  sulphides  is  so  marked  in  the  disseminated  ores  at  Engels 
and  at  Copper  Mt.,  that  these  deposits  are  properly  classed 
as  ores  of  hydrothermal  origin  rather  than  pneumatolytio. 

In  all  these  deposits,  however,  it  is  fairly  certain 
that  the  earliest  formation  of  the  bornite  took  place  under 
pneumatolytic  conditions.  At  Evergreen  and  La  Fleur  Mt., 
the  chief  mineralization  was  confined  to  this  period  but  at 
Engels  and  Copper  Mt.,  the  formation  of  the  sulphides  con- 
tinued and  became  of  even  greater  importance  under  succeeding 
hydrothermal  conditions. 

The  absence  of  pyrite  in  deposits  of  the  magmatic- 
pneumatolytio  type  (including  Ookiep)  is  noteworthy.  Magnetite 
is  usually  abundant  (except  at  Evergreen j.  In  all  cases  it 
is  the  earliest  ore-mineral,  and  indicates  that  the  deposition 
of  sulphur  was  unimportant  in  the  early  phases  of  the  miner- 
alization. The  association  of  sulphur  with  the  increasing 
copper  suggests  that  it  may  have  played  an  important  part  as 
a  mineralizer. 

1.  Tolman  and  Rogers  (loc.  oit.,  p.  15,  footnote  25)  con- 
sider sulphur  as  a  aineralizer  of  importance  in  the  magmatio 
stage  in  this  type  of  deposit. 


. 
i  »3lio  nation-. 

<{l&«*ol&  *ao(B  «j*w  & 

• 

238     «OdC 

•olo  e 

erf?  1 

js  ca  si  aafcldql 
e: 


-  1  jfl^ift  ' 


-  1  jfl^ift  ' 
a   flWXSlWl    *A       ,8«Ol*iJ>nOO    Qt.$TtlQ8&ft^ 

•j  ««;w  nol^sllassfilw  %»1 

s'i    «i:  lx 


J«rffi 

- 

. 

•     tofiJ  liOi/   SJg* 


i?   > 


319. 


The  praotioal  absence  of  pyrite  and  the  dominance 
of  magnetite  in  the  early  phases  of  the  mineralization  in 
these  deposits  rcay  be  interpreted  as  an  indication  either 
that  sulphur  was  not  present  in  notable  concentrations  at 
the  start  but  was  introduced  later  with  the  copper,  or  that 
the  temperature  preceding  the  deposition  of  the  copper  was 
above  the  dissociation  point  of  pyrite.  The  dissociation- 
pressure  curve  of  pyrite,  rsoe.-tly  established  by  Allen  and 

1 
Lombard   indicates  that  although  pyrite  is  practically 

stable  up  to  about  600°C.,  beyond  that  point  the  sulphur 
vapor  pressure  necessary  to  prevent  the  dissociation  of 
pyrite  increase*  very  rapidly  with  the  temperature.  It  sug- 
gests that  if  the  dissociation  interpretation  is  accepted, 
and  if  the  conditions  of  the  dissociation  experiments  may 
be  applied  directly  to  the  conditions  in  nature,  then  the 
temperatures  at  which  the  magnetite  formed  were  not  below 
700°C. 

Somite  in  Deposits  of  Con  tao  t-  Met  amor  phio  Origin 

Bornite  ores  produced  by  oontaot-metamorphio  ac- 
tion may  be  divided  into  two  classes: 

1.  the  ores  associated  with  intense  localized 
alteration  of  invaded  sediments  and  intrusive 
along  the  contact;  and 

2.  the  ores  associated  with  a  widespread  develop- 
ment of  hydrothernal  minerals  such  as  epidote, 
chlorite,  and  sericite,  in  both  the  county  rook 
and  the  igneous  rock. 

1.  A  method  for  the  Determination  of  Dissociation  Pressures 
of  sulphides  and  its  application  to  oovellite  (Cue)  and  py- 
rite (FeS9),   A.  J.  S.,Vol.  43.  p.  193.   (1S17). 


• 


330. 


The  ore-bodies  of  the  first  group,  auoh  as  the  Warble  Bay, 
Whitehorse  and  Seven  Devils  deposits,  are  usually  smaller, 
and  nay  be  considered  of  direct  contact -metamorphio  origin 
in  the  strictest  usage  of  the  word.  The  ores  of  the  second 
group,  which  are  of  greater  commercial  importance,  and  are 
well  illustrated  by  the  extensive  deposits  at  Bisbee,  seen 
to  be  the  product  of  milder  conditions  that  way  extend  far- 
ther from  the  igneous  mass  and  in  which  contact- me tamorphic 
conditions  may  grade  without  break  into  hydrothermal.  The 
source  of  the  netals  is  clearly  the  intrusive  rook  in  both 
oases,  but  in  the  latter  the  distribution  of  the  ores  is 
likely  to  be  less  closely  related  to  the  actual  contact  than 
in  the  former. 

In  the  three  deposits  of  the  first  type  which 
have  been  studied,  bornite  is  the  chief  ore-mineral,  but 
in  each  it  is  associated  with  almost  equal  amounts  of  ohaloo- 
pyrite.  In  part  it  is  contemporaneous  with  the  ohaloopyrite 
in  period  of  formation,  but  probably  continued  to  form  some- 
what longer.  Both  sulphides  are  later  than  magnetite,  hema- 
tite or  pyrite.  In  these  deposits  pyrite  is  an  unimportant 
mineral  and  the  early  deposition  of  iron  usually  tales  the 

form  of  magnetite  and  speoularite.  The  similarity  of  contact 

to  the  group  previously  described _ 
rr.etarr.orphic  deposits,  suggests  that  the  initial  conditions 

under  which  deposition  took  place  in  the  former  were  not  far 
rarroved  from  the  early  phases  of  deposition  of  the  ore- 
minerals  in  magmatio-pneumatolytio  deposits.  The  occurrence 


-Ti.- 

- 

\*[k  '3  sae 

. 

••BC 


J<J    99  it 

. 


331. 

of  iron  oxides  rather  than  pyrite  ia  probably  susceptible  of 
the  sane  explanation  as  mentioned  on  page  319  .  The  similari- 
ty in  this  feature  to  the  ores  of  rcagcatio  origin  suggests 
that  the  original  conditions  of  metallic  deposition  in  the 
contact -rcetamor phi o  ores  were  of  an  intensity  comparable  to 
those  in  the  initial  stages  of  deposition  under  zcagrratio 
and  pueuaatolytio  conditions.   It  is  possible  that  the 
temperatures  in  the  early  stages  are  above  those  at  which 
sulphur  can  remain  united  with  iron  as  pyrite.  Where  pyrite 
is  abundant,  as  in  the  Bisbes  ores,  magnetite  is  usually  of 
little  importance .  The  possible  importance  of  sulphur  as 
a  mineral! zer  associated  with  the  copper  is  again  emphasized. 

The  ore  minerals  are  all  later  than  the  high- 
temperature  silicates  of  the  garnet  zone.  In  the  three  de- 
posits studied,  the  ores  show  a  distinct  tendency  to  occur 
between  intensely  altered  rook  on  one  side,  and  unaltered 
rock  on  the  other,  a  fact  which  supports  Umpleby's  generaliza- 
tion that  in  c ont act -met amor phio  deposits  the  ore  usually 
occurs  on  the  limestone  side  of  garnet  zones.  Under  the 
microscope,  the  sulphides  may  be  seen  to  be  later  than 
garnet,  diopside,  vesuvianite,  and  epidote,  but  more  closely 
associated  or  contemporaneous  with  chlorite  and  sericite, 
and  followed  by  carbonates  and  in  one  case  (Marble  Bay, 
Texada  Island)  by  zeolites. 

From  these  relations,  it  is  evident  that  the  bornite 
does  not  occur  under  the  intense  early  phases  of  the  mlneral- 

1.  Ice.  cit. 


. 

eeoi 

. 

.»;    adgi.*  3   \1  •£*•,&   ® 
, 

':*'j«u 

T      .  «Di':'*3ioqtai   e 

•  xlig .'  ' 

®6  ."   lo^e^oiXis 

. 

-        '  ua  rfoiriw  jo^ 

<*fc   Oi 


a&v   , 
-  .  njnoqaifc.i 


. 


332. 


i sat ion,  but  is  a  product  of  conditions  of  increasingly 
hydrothermal  tendencies  along  with  chlorite  and  sericite, 
finally  dying  out  as  the  conditions  become  favorable  for 
the  formation  of  carbonates  and  zeolites. 

At  Bisbee,  where  bornite  is  an  important  ore- 
aineral,  although  in  general  less  abundant  than  chaloopyrite, 
the  sarre  general  relations  hold,  except  in  different  degrees. 
The  high  temperature  silicates  again  are  earlier  than  the 
ore -mineral s,  but  they  are  developed  only  in  a  narrow  contact 
zone.  According  to  the  work  of  Bonillas,  Tenney  and  Feu- 
chere,   the  ohalcopyrite  and  bornite  are  later  than  the 
aerioite,  and  are  most  commonly  accompanied  by  widespread 
siliciftoation.  This  relation  and  the  occurrence  of  the 
sulphides  in  almost  unaltered  limestone  suggest  that  in 
deposits  of  this  type  the  formation  of  bornite  takes  place 
under  similar  or  milder  conditions  than  in  the  deposits  of 
the  direct  cont act -met amor phi c  type. 

Bornite  in  Deposits  of  Hvdrothermal  Origin 

It  has  been  apparent  in  the  discussion  of  deposits 
under  other  headings  that  a  portion  of  the  bornite  in  nearly 
all  cases  is  associated  with  minerals  of  hydrothermal  origin, 
and  should  be  assigned  to  that  genetic  class.  Consequently, 
there  remain  for  discussion  here,  only  the  deposits  of  simple 

1.  loo.  oit. 


1 


»-  *JElSI    f£Tftn0J.. 

lie   •m/tfjree-.ifflBJ 
at*  ^fe  -si 

.  ftflO 


333. 


hydro-thermal  character,  which  lack  those  interesting  features 
such  as  pneumatolytic  products  or  dependence  on  igneous  con- 
tacts which  would  bring  them  under  discussion  elsewhere. 
The  veins  of  sulphides  in  unaltered  limestone  at  Kenneoott 
are  believed  to  be  of  hydrothermal  origin,  but  on  account  of 
the  many  puzzling  features  presented,  they  have  been  treated 
under  a  separate  heading. 

The  deposits  of  this  group  in  which  bornite  is  an 
important  mineral  are  chiefly  vein  deposits.  The  ores  are 
usually  accompanied  by  the  development  of  sericite,  chlorite 
and  to  a  less  extent  epidote  and  carbonates  in  the  wall  rock, 
and  quartz  in  the  veins  themselves  or  in  the  immediate  walls. 

Pyrite  and  magnetite  (as  at  the  Superior  Mine, 
Engels)  are  invariably  earlier  than  the  copper  minerals.  The 
pyrite,  which  is  abundant  at  Magma  and  at  Butte,  is  apparent- 
ly bre,ooiated  and  reoeir.ented  by  chaloopyrite,  bornite,  and 
the  later  minerals,  but  it  is  probable  that  the  actual  frac- 
turing which  made  channel-ways  for  the  later  phases  of  the 
mineralization  was  very  slight,  and  that  the  breooiated 

appearance  is  largely  due  to  the  character  of  the  replace- 
to 

zr,ent  andAstruotuzal  properties  of  the  pyrite.  In  tha  Magma 
deposit,  which  offers  a  simple  and  fairly  typical  example 
of  a  normal  hydrothermal  deposit,  the  pyrite  is  accompanied 
by  silicifioation  and  serioitization  of  the  wall-rock.   The 
development  of  quartz  continued,  however,  with  the  deposition 
of  chaloopyrite  a:.d  bornite.  The  formation  of  chlorite  in 


t  Jb 


. 

rtfXSJ    ( 

.n  Jbo*  ©d 


( 


384. 


the  county-rook  probably  accompanied  this  phase  of  the  min- 
eralization. Carbonates  ±a  usual  are  later  product a. 

The  complex  relations  at  Butto  have  not  been  con- 
sidered in  detail  in  connection  with  this  work,  for  they  are 
being  studied  intensively  at  present  as  a  separate  problem 
by  another  investigator.  In  general,  however,  it  nay  be  said 
that  the  primary  mineral  associations  of  the  gangue,  altera- 
tion products  and  ore-minerals  indicate  that  bornite  is 
formed  under  conditions  of  intermediate  intensity,  similar 
in  a  broad  way  to  the  simpler  relations  at  Magma  and  else- 
where . 

In  the  ore -deposit  at  the  slightly  developed 
Superior  Uine  near  Engels,  California,  the  mineralization 
associated  with  the  bornite  indicates  an  unusually  wide  range 
in  conditions  of  formation  for  this  type  of  deposit.  The 
earliest  minerals  in  the  wall  rock  and  the  veins  are  albite, 
tremolite  and  biotite,  all  indicating  fairly  high  tempera- 
tures. The  early  deposition  of  iron  takes  the  form  of  mag- 
netite. Tha  copper  follows  with  sulphur,  in  part  late  and 
in  part  associated  with  epidote,  chlorite,  and  sericite. 
The  opening  stages  of  the  mineral  deposition  at  Superior 
are  close  to  the  border  line  between  pneumatolytio  and  hy- 
drotheraal  conditions.  The  continuation  of  the  processes 
of  mineralization  under  mild  conditions  is  indicated  by  the 
occurrence  of  heulandite  in  notable  amounts.  Ordinarily 
in  bornite  deposits  of  hydrothermal  origin,  the  various  min- 


335. 


erals  are  more  closely  related  in  conditions  of  formation, 
although  the  mineralization  Buy  have  been  longer  oontinued 
and  greater  in  extent. 

The  bornite-ohaloopyrite  deposits  in  basio  lavas 
afford  a  special  class  of  hydrothermal  deposits  different 
in  origin  and  different  in  many  details  from  the  preceding 
deposits.  The  deposits  in  the  Nikolai  greenstone  in  the 
Kenneoott  district  are  the  only  ones  of  this  type  which  have 
been  studied  in  connection  with  this  investigation,  but  the 
constancy  of  features  which  this  type  of  ore-deposit  shows 
gives  the  treatment  presented  on  previous  pages  a  general 
value.  The  evidence  indicates  that  the  ores  were  concen- 
trated from  the  basio  lavas  themselves,  in  part  perhaps  by 
means  of  emanations  closely  following  their  extrusion,  but 
in  sorre  cases  very  probably  by  the  circulation  of  heated 
waters  through  the  flows  under  later  conditions.  The  veins 
are  rarely  large  or  persistent,  but  the  mineralisation  is 
widely  distributed.  The  characteristic  gangue  is  quartz, 
carbonates  and  epidote,  with  widespread  development  of 
chlorite  in  the  wall-rock.  Albite,  datolite  and  zeolites 
are  also  formed  in  association  with  the  sulphides,  but  are 
usually  not  abundant.  Bornite  is  later  than  the  quartz, 
epidote  and  ohlorita,  but  in  part  earlier  than  the  oarborate 
It  is  later  than  pyrite,  and  although  closely  associated 
with  chalcopyrite,  it  lags  somewhat  behind  as  usual. 


HKf    9V. 

.-.loeai 


&:V 

*  2-      .  .o  b»jn»- 

• 


•Afts  jlfi  IdO[     1O 


326. 


Somite  in  Re  place  merit  Ores  in  Unaltered  Limestone 

The  only  deposits  of  this  type  which  have  been 
considered  are  the  ores  in  the  Chitiatone  limestone  at 
Kenneoott.  The  primary  features  of  these  ores  have  been 
discussed  in  great  detail,  and  will  not  be  reconsidered  here. 
The  primary  ores  are  believed  to  have  been  formed  by  ascend- 
ing heated  solutions  which  had  derived  their  copper  content 
frou.  the  underlying  greenstone  formation.  The  chief  prob- 
lem in  connection  with  the  Kennecott  ores  is  the  primary  or 
secondary  nature  of  the  chalcocite.  The  critical  features 
depend  upon  the  interpretation  of  the  alteration  of  bornite, 
and  will  be  discussed  under  that  heading. 

Bornite  Deposited  from  Cold  Meteoric  Solutions 

Ores  of  the  "Red  Be  IB"  Type.   Bornite  occurs  in 

8tBa.ll  amounts  in  ores  of  the  "Red  Beds"  type  in  various  parts 

1 
of  the  world.    Chalcooite  is  the  usual  sulphide  in  deposits 

of  this  sort,  but  in  many  districts  it  is  associated  with 
arca.ll  amounts  of  hematite,  pyrite,  bornite,  ohaloopyrite, 
and  covellite,  accompanied  by  malachite,  azurite,  limonite 
and  other  products  of  oxidation.  The  ore-minerals  are  com- 
monly replacements  in  'trie  sedimentary  rooks,  usually  at- 


1.  For  bibliographies  of  deposits  of  this  type  and  dis- 
cussion, see  Liridgren'a  Mineral  Deposits,  pp.  363-376;  for 
European  localities,  Stelzner  and  Bergeat,  Die  Erzlager- 
s tat ten,  1904,  pp.  338-439. 


'^fftffl  { 


337. 


tacking  the  cementing  material  or  eatoicbed  carbonaceous  nat- 
ter. Where  sulphides  other  than  ohaloooite  occur,  the  ore- 
minerals  are  usually  in  sequence,  the  earlier  being  partially 
replaced  by  the  later. 

Excellent  microphotographs  of  wood  structure  re- 
placed by  hematite,  pyrite,  bornite  and  ohaloooite  have  been 

•> 

4 

published  by  A.  F.  Rogers.    In  material  from  Sierra  Oaoura, 
New  Mexico,  it  is  clearly  shown  that  the  bornite  was  derived 
in  part  at  least  by  the  replacement  of  pyrite,  and  the  ohal- 
oooite by  replacement  of  the  bornite.  A  little  chaloopyrite 
and  oevellite  are  also  found  as  replacements  of  bornite. 
The  replacement  of  hematite  by  pyrite,  the  two  ages  of  hema- 
tite, and  the  climatic  changes  deduced  by  Rogers,  are,  how- 
ever, difficult  to  follow. 

The  question  of  the  origin  of  ores  of  the  "Red 

2 
Beds"  type  cannot  be  regarded  as  finally  settled,   but 

most  writers  are  in  agreement  that  in  general  the  sulphides 
were  deposited  from  cold  meteoric  solutions. 

Bornite  noiules  i:t  the  lower  beds  of  the  Chitis%one 

3 
lirr.estone  near  Kenneoott,   and  in  shales  in  Uashonaland, 

4 
southern  Rhodesia,  described  by  F.  P.  Mennell,   are  most 


1.  Origin  of  Copper  Ores  of  the  "Red  Beds"  type,  Eoon. 
Geol.,  Vol.  II,  pp.  366-380,   (1816). 

2.  The  Sierra  Osoura  ores  and  others  of  the  "Red  Beds" 
type  have  been  attributed  to  heated  ascending  solutions  by 
L.  C.  Graton,  Prof.  Paper,  No.  68,  U.S.G.S.,  (1910). 

3.  p.  261. 

4.  Minaralogical  Magazine,  Vol.  17,  p.  Ill,   (1914). 


-3*1. 


.-.iv  93  s.n'ax.  . 

• 


)*f' 


>  d<J 
O3ie«nsjs  ni  S-XA  ?. 

so 


388* 


probably  to  be  attributed  to  meteoric  agencies. 

Secondary  Bornite.   Contrary  to  general  belief, 
bornite  is  very  rarely  of  secondary  origin  in  sulphide  de- 
posits. As  has  been  emphasized  previously  in  many  places, 
bornite  is  frequently  a  replacement  of  earlier  ore -mineral a, 
such  as  iragnetite,  pyrite,  enargite.or,  to  a  slight  extent, 
chaloopyrite,  but  the  field  relations  rcake  it  very  apparent 
in  all  oases  studied,  that  the  sequence  of  minerals  was  due 
to  primary  causes  and  in  no  way  related  to  the  processes  of 
secondary  enrichment.  The  deposition  of  bornite  ores  in 
general  shews  no  variations  with  the  factors  which  control 
the  development  of  secondary  sulphides,  and  it  is  safe  to 
conclude  that  the  distribution  of  rich  bornite  ores  will 
not  be  limited  to  the  range  of  surface  agencies. 

Bornite  of  secondary  origin,  however,  has  been 
observed  in  microscopic  quantities  in  many  deposits.  At 
Globe,  small  ve inlets  of  bornite  cutting  chaloopyrite  areas 
seem  clearly  associated  with  the  development  of  ohaloooite 
and  are  probably  of  similar  origin.   In  the  Bisbee  ores  fine 
veinlets  of  ohaloooite  in  bornite  have  been  observed  to  con- 
tinue as  veinlets  of  bornite  when  a  chaloopyrite  area  was 
intersected. 

The  "halos"  of  bornite  round  ohalcopyrite  grains 
in  a  field  of  ohaloooite  are  most  readily  interpreted  to  be 
an  intermediate  step  in  the  alteration  of  ohalcopyrite  to 
ohalcooite.   In  ores  from  the  Magma  Mine  this  structure  is 


a  • 


£•   lo 


339. 

well  illustrated.   Secondary  Somite  of  this  sort  has  also 
been  observed  under  the  microscope  in  many  of  the  porphyry 
ores,  at  tnose  of  3righam;Ray,  Morenoi,  and  Ely.  In  all 
these  cases,  however,  the  quantity  of  the  secondary  bornite 
is  almost  negligible,  when  compared  with  the  total  sulphide. 

Pyrite  grains  in  chalcooite,  where  the  enriching 
changes  are  well  advanced  and  probably  intense,  are  fre- 
quently observed  to  be  surrounded  by  thin  rims  of  ohalco- 
pyrite  which  are  in  turn  bordered  by  a  similar  rim  of  bornite 
forming  composite  "halos".  In  general  this  relationship  is 
to  be  interpreted  as  a  result  of  the  alteration  of  pyrite 
by  enriching  solutions,  and  the  ohalcopyrite  and  bornite 
are  merely  intermediate  steps  in  the  transformation  of  py- 
rite to  chaloooite.  An  attaok  on  pyrite  by  the  enriching 
solutions  under  conditions  unfavorable  for  the  removal  of 
iron  would  probably  be  the  best  suited  to  yield  the  inter- 
mediate products.  Halos  of  this  sort  have  been  observed  in 
secondary  ores  from  a  number  of  the  camps  just  mentioned  and 
give  very  certain  evidence  of  the  partial  replacements  of 
pyrite. 

At  Butte,  bornite -chalcopyrite  halos  have  been  ob- 
served in  ores  from  almost  all  depths  explored.  For  the 
most  part  they  are  identical  with  the  relations  noted  in 

secondary  ores,  but  ohalcopyrite  halos  have  been  observed 

1 
about  pyrite  grains  set  in  a  wide  field  of  bornite   which 

1.  D.  A.  Hall,  Private  communication,  Febr.  1917. 


-313 

»Cf    Qi 


• 


e. 


,  fcd*=. 


330. 

cannot  with  certainty  be  ascribed  to  secondary  agencies. 

Summary  of  Conditions  under  Which  Somite  Hag  Been  Formed 

Bornite  forma  an  important  ore  in  only  one  well 
known  deposit  of  the  direct  magmatio  type  (Ookiep).  In  this 
case,  the  bornite  is  the  last  of  the  series  of  rock  and  ore- 
minerals,  arid  was  probably  deposited  under  late  oagn&tio  con- 
ditions. The  initial  segregation  of  the  copper  is  attributed 
to  direct  magmatio  differentiation,  but  the  final  adjustments 
and  concentration  of  the  sulphides  into  ore-shoots  is  be- 
lieved to  have  been  caused  largely  by  the  action  of  nineral- 
izers.  The  temperature  under  which  the  bornite  was  formed, 
is  much  lower  than  that  at  which  the  rook  minerals  usually 
separate,  and  probably  is  comparable  to  the  temperatures 
prevailing  during  the  crystallization  of  pegmatites. 

In  several  deposits,  bornite  forms  a  primary  con- 
stituent of  pegmatites,  or  differentiated  dikes  of  soitewhat 
similar  origin,  but  here  again  it  is  one  of  the  latest  min- 
erals to  have  been  deposited,  and  was  probably  formed  at 
temperatures  lower  than  that  commonly  assigned  for  the 
crystallization  of  pegmatitio  quartz,  in  the  neighborhood 
of  575°C.  The  bornite  in  pegmatites  is  rarely  abundant 
enough  to  give  promise  of  becoming  of  commercial  value. 
Rich  bornite  ores  occur  in  several  well  known 
contact-ite  tamorphic  deposits.  Again  the  mineral  is  clearly 
later  than  all  high  temperature  products;  it  replaces  earlier 


331. 


contact -me  tairsorphio  silicates  auoh  as  garnet,    diopside, 

the 

veauvlanite,  and  is  the  latest  important  sulphide  of  prima- 

A 

ry  sequence. 

From  the  study  of  ores  of  the  contact -me  tairorphic 
type,  and  also  of  ores  in  veins  of  hydrothermal  origin, 
there  is  strong  evidence  that  bornite  in  greatest  quantities 
is  formed  under  hydrothermal  conditions,  usually  a  little 
later  than  the  period  characterized  by  the  formation  of  py- 
rite  and  epidote,  and  about  contemporaneous  with  the  most 
intense  development  of  serioite  and  chlorite.  The  formation 
of  bornite  ceases,  or  continues  in  a  diminishing,  feeble  way, 
under  later  conditions  which  are  characterized  by  the  develop* 
sent  of  carbonates  or  zeolites. 

The  penetration  of  bornite  grains  by  serioite  or 
chlorite  laths  has  been  interpreted  by  A.  F.  Rogers  as  a 
proof  that  the  sulphides  are  replaced  by  these  silicates. 
The  relation  has  been  observed  in  ores  from  many  districts 
in  the  course  of  this  work.  In  nearly  all  cases,  the  evi- 
dence is  capable  of  two  interpretations,  viz.  that  of  Rogers 
and  the  commoner  view  that  the  silicates  are  inclusions  in 

the  sulphides.  Ve inlets  of  serioite  or  chlorite  have  not 

1 
been  observed  cutting  bornite  or  ohalcopyrite  areas.    At 

Sngels  there  is  evidence  that  the  chlorite  was  produced  to- 


1.  It  should  be  noted  that  the  ore-minerals  out  by  veinlets 
of  chlorite  shown  in  Tolman  and  Rogers'  photographs  of  Engela' 
ores  (loo.  oit.  fig.  77),  are  magnetite  and  hematite  and  not 
sulphides.  This  is  not  mentioned  in  the  text.  (p. 63.) 


obiva 


' 

a^ 

©    Odj 


333. 

ward  the  close  of  the  bornite  forming  period,  but  the  in- 
timate relations  with  the  sulphide*  and  the  distinct  examples 
of  oorroaion  of  chlorite  by  bornite  are  difficult  to  explain 
under  Rogers'  interpretation.  Except  for  the  absence  of 
notable  corrosion,  the  relations  of  the  sericite  to  bornite 
are  the  same  as  those  of  the  chlorite.   Summarizing  the  re- 
lations observed  in  the  districts  studied  in  the  course  of 
this  work,  the  conclusion  that  the  development  of  sericite 
and  chlorite  usually  preceded  or  was  contemporaneous  with 
the  formation  of  bornite  harmonizes  the  evidence  most 
satisfactorily. 

Bornite  with  associated  ohaloopyrite  in  veins  in 
basic  lavas  affords  another  example  of  the  hydrothermal  orig- 
in of  the  mineral.  This  type  of  deposit,  however,  ia  rarely 
of  commercial  importance.  The  Kennecott  deposits  in  lime- 
stone also  constitute  an  example  of  bornite  ores  formed  under 
relatively  mild  conditions  of  pressure  and  temperature  al- 
though probably  from  heated  solutions. 

Small  quantities  of  bornite  in  ores  of  the  "Red 
Beds"  districts,  and  occasional  nodules  in  impure  limestones 
or  shales  indicate  that  the  mineral  may  bs^  capable  of  forma- 
tion from  meteoric  waters* 

The  occurrence  of  secondary  bornite  at  a  replace- 
ment of  primary  sulphides  by  the  processes  of  surface  enrich- 
ment, gives  positive  •Widenoe  that  bornite  can  be  produced 
in  cold  solutions.  The  amount  of  secondary  bornite,  how- 


.   .,  . 


siec 


. 


333. 


ever,  ia  very  aaall.  It  is  rarely  observed  except  under  the 
microscope. 

It  is  apparent  from  the  preceding  summary  that 
bornite  forrra  under  an  usually  great  variety  of  conditions* 
The  deposits  in  which  it  occurs  range  in  origin  from  late 
magc-atic  to  feeble  hydr  other  rral,  and  also  include  those 
formed  by  cold  meteoric  agencies. 

In  deposits  commonly  classed  as  oagrratio,  bornite 
is  uncommon;  it  is  of  wider  distribution  in  deposits  of  the 
pneuaaatolytio  type,  although  rarely  in  large  bodies;  it  is 


Fig.  ^3.   Diagram  of  Distribution  of  Bornite... 

Curve  I.   Distribution  of  boruite  based  on  classification 

of  coatainin.? 


. 

Curve  II.   Distribution  of  bornite  oased  on  conditions  of 
forrjation  of  bornite  itself.. 


(  8  •: 


K-J 
rv 


334. 


of  widest  distribution  and  greatest  commercial  importance 
in  veins  and  other  replacements  classed  aa  hydrotherroal  de- 
posits; it  is  only  iu  very  snail  and  unimportant  amounts 
in  deposits  froir  cold  meteoric  solutions,  including  seconda- 
ry sulphide  ores.   Curve  I  of  Fig.  39  illustrates  this  dis- 
tribution of  bornite  in  a  qualitative  way  in  the  different 
classes  of  deposits.  As  has  been  mentioned,  however,  the 
bornite  in  most  of  the  deposits  is  later  than  the  group  of 
minerals  —  among  which  gangue  minerals  are  given  greatest 
weight  —  upon  which  the  classification  is  baaed.  Conse- 
quently the  sunraary  of  conditions  under  which  the  bornite 
itself  is  actually  formed  is  believed  to  be  represented  more 
31    accurately  by  Surve  II  of  Fig. 39   than  by  Curve  I.  The 

relative  abundance  of  bornite  under  the  different  conditions 
is  indicated  by  the  height  of  the  curves. 


335. 


THE  ALTERATION  OF  BORNITE 
Introduction 

In  most  of  the  ore-bodies  described  on  previous 
pages,  the  formation  of  the  primary  sulphides  ceased  with 
the  close  of  the  period  of  bornite  deposition.  Here  and 
there,  a  little  chalcopyrite,  galena,  tennantite,  or  tetra- 
hedrlte  may  have  continued* to  form  in  a  feeble  way,  as  a 
line  of  fine  blebs  or  an  indefinite  veinlet,  but  for  the 
most  part,  the  only  sulphides  which  occur  in  abundance  later 
than  the  bornite,  are  those  included  in  the  group  of  which 

chaloocite  is  the  most  prominent  member.   In  the  majority 

. 

of  cases,  the  chalcooite  and  the  related  minerals  may  be 
easily  recognized  as  products  of  the  superficial  alteration 
of  the  ore-body,  but  in  some  deposits,  where  they  persist 
to  great  depths,  or  where  certain  peculiar  relationships  to 
the  other  sulphides  exist,  it  may  be  difficult  to  determine 
whether  they  are  the  result  of  secondary  processes  or  are 
the  final  members  of  the  series  of  primary  minerals* 

Field  Relations. 

The  field  relations  afford  conclusive  evidence  of 
the  secondary  origin  of  the  ohalcooite  in  many  cases,  especially 
where  the  enrichment  is  confined  to  a  limited  cone  beneath 


356. 

the  oxidised  ores,  and  is  succeeded  below  by  leaner  sul- 
phides which  show  little  change  with  increasing  depth. 
These  relations  are  usually  more  stron-ly  marked  in  de- 
posits where  pyrite-chaloopyrite  ores  constitute  the  pri- 
mary sulphides,  than  where  bornite  is  an  important  constitu- 
ent. With  the  same  depth  of  ore  exposed  to  oxidation,  it 
is  evident  that  less  copper  is  made  available  for  the  en- 
riching solutions  in  the  oase  of  the  pyrltio  ores  than  when 
bornite  ores  are  oxidized*  This  and  the  greater  addition 
of  copper  necessary  to  convert  pyrite  and  chaloopyrite  to 
ohaloocite  than  to  accomplish  the  same  change  in  a  bornite 
ore,  tend  to  limit  the  enrichment  to  a  narrower  and  more 
definite  zone  in  the  oase  of  the  pyritio  deposits* 

The  results  of  a  calculation  in  a  simple,  ideal- 
ized oase  emphasize  this  point.  Assume  for  a  basis  of 
comparison  two  ores,  one  containing  10$  bornite  by  volume, 
and  the  other  10$  chalcopyrite,  both  in  an  inert  g&ngue. 
The  bornite  ore  would  assay  11.1  per  cent  copper,  the 
chaloopyrite  ore  5.30  per  cent.  Assume  that  the  deposits 
are  in  a  vertical  position,  and  that  oxidation  penetrating 
to  a  depth  of  100  feet,  had  completely  converted  the  copper 
into  the  soluble  sulphate,  which  descended  into  the  sulphides 
beneath,  and  had  reacted  with  them  according  to  the  equation 

5  Cu5FeS4  -I-  11  CuS04  -f  8  H20 

18  Cu2S  +  5  FeS04  -f-  8  E,S04 
in  the  oase  of  the  bornite,  and  according  to  the  equation:  — 


. 


337. 

5  CuFeSg  +  11  Cu  S04  -h  6  H20    * 
8  CugS  -t-  5  FeS04  -»-  8  HgSO^ 

in  the  oaee  of  the  chalcopyrite.1  If  the  replacement  of  the 
primary  sulphides  by  chulcooite  was  complete  ae  far  as  the 
influence  of  the  cupric  sulphate  extended,  the  bornite  would 
be  converted  to  chalcocite  for  a  depth  of  236  feet  below  the 
bottom  cf  oxidation,  and  the  ohaloopyrite  to  a  depth  of  45.3 
feet.  The  volume  changes  involved  on  the  basis  of  these 
equations  are  almost  negligible ,  The  ore  would  be  increased 
from  11.1/i.  copper  to  13.6$  copper  in  the  case  of  the  bornite 
deposit,  and  from  5.3$  copper  to  15.6$  copper  in  the  oaee  of 
the  chalcopyrite  deposit,  but  in  the  bornite  deposit  the  en- 
richment would  extend  five  times  as  deep  as  in  the  other* 

Under  natural  conditions,  only  part  of  the  original 
copper  in  the  oxidized  zone  would  be  available  for  the  en- 
riching processes.  Vary  in,;;  amounts  would  remain  fixed  as 
carbonates,  silicates  and  oxides.  Losses  of  copper  are  in- 
evitable in  the  movement  of  the  sulphate  solutions.  While 
these  factors  would  reduce  the  thickness  of  the  ohalcocite 
zone  and  the  depth  of  enrichment,  they  would  tend  to  be 
balanced  by  the  fact  that  at  many  places  the  enrichment 
would  be  only  partial,  and  therefore  that  some  ohalcocite 
would  be  formed  deeper  than  indicated  in  the  ideal  case. 


1.  E.  0.  Zeie,  E.  T.  Allen,  and  H.  E.  Merwin,  Secondary 
copper  sulphide  enrichment^  Boon.  Geol.,  Vol.  II, ~ 

bQO  f     (  1 :;  io  )  • 


H- 

338. 

Although  the  actual  depths  indicated  in  the  oomputation  axe 
of  little  value,  the  ratio  of  the  depth  of  enrichment  in  a 
bornite  ore  to  the  depth  of  enrichment  of  a  ohalcopyrite  ore 
is  probably  a  close  expression  of  the  conditions  which  actu- 
ally obtain  in  nature.  The  contrast  in  depth  of  enrichment 
between  lean  pyritic  ores  and  bornite  ores  is  even  more 
striking  than  in  the  case  calculated  in  which  a  rich  ohaj.oo- 
pyrite  ore  was  assumed,  for  in  such  lean  deposits  not  only 
is  there  less  copper  made  available  by  a  given  depth  of  oxi- 
dation and  leaching,  but  also  a  given  quantity  of  oopper  will 
produce  a  much  smaller  quantity  of  chalcocite*  In  the  caae 
of  the  pyrltlc  deposits,  the  change  in  value  from  the  second- 
ary ore  to  the  primary  ore  is  much  greater  than  in  the  bornite 
deposits,  but  the  development  of  the  secondary  sulphides  ie 
far  more  extensive  in  the  latter  under  similar  conditions. 
From  the  preceding  considerations  it  is  apparent 
that  while  the  field  relations  usually  give  definite  in- 
formation concerning  the  secondary  or  primary  nature  of 
chalcocite  ores  in  pyritic  deposits,  baaed  on  their  behavior 
with  depth,1  the  evidence  in  the  case  of  deposits  in  which 
bornite  is  an  important  primary  constituent,  may  be  more 
difficult  to  interpret.  If  the  enrichment  in  bornite  oras 
is  confined  to  a  shallow  zone,  the  secondary  origin  of  the 
ohaloooite  is  even  more  definitely  eetabliehed  than  in  the 


1.  See  A.  Locke  and  E.  H.  Perry,  Interpretation  of 
Assay  Curves  or  Drill  Holes,.  Trans .  A.I.H.E.,  Vol.    , 
p.  93-99, 1916 . 


BtfUfOl' 


••'*    <*  S*    i 

' 

. 

I 

•   s^at-T 


- 

. 
-• 

*d$  «eiroc 

5I1Q      f- 

• 

. 

.-i;    8TOB   «6V^>    lii^JtO 


339. 


case  of  similar  relations  in  pyritic  ores,  but  in  none  of 
the  bornite  ores  which  have  been  studied,  does  the  continu- 
ance of  the  chaloocite  with  depth  alone  establish  the  primary 
nature  of  the  replacement.  The  possibility  cannot  be  denied 
that  the  enrichment  of  bornite  by  surface  agencies  may  take 
place  under  favorable  circumstances  as  deep  as  any  of  the 
ohalcocite-bornite  ores  observed  in  the  course  of  the  work 
of  the  Secondary  Enrichment  Investigation. 

Selective  Enrichment* 

The  relative  ease  with  which  the  change  from 
bornite  to  chaloocite  takes  place  is  an  important  factor  in 
explaining  the  features  of  enriched  bornite  ores*  The  common- 
est ore-minerals  associated  with  bornite  may  be  placed  in  the 
following  sequence  with  respect  to  their  resistance  to  altera- 
tion to  chalcocite:  pyrite,  enargite,  sphalerite,  ohalcopyrite, 
bornite.  Between  the  pyrite  and  enargite  may  be  grouped 
tennantite  and  tetrahedrite,  and  between  the  chalcopyrite  and 
bornite,  galena  should  be  placed,  but  the  evidence  fixing 
the  positions  of  these  members  of  the  series  is  not' entirely 
satisfactory,  and  their  exact  places  are  not  definitely  es- 
tablished. The  information  by  which  the  relative  ease  of 
replacement  by  chalcocite  is  established  is  gained  in  small 
part  from  field  observations,  but  chiefly  from  the  relations 
observed  under  the  reflecting  microscope  which  alone  are 
really  reliable. 


-cKi-sTfroe  taj  £  .orci/ys 

iq  erf*  -fai  r 

. 


'10  JO* 


'3     Oi1     ? 


•rtei 
r(e 

:!•» 

.:xe  i-: 


340. 

Evidence  such  as  that  afforded  by  the  disseminated 
ores  in  Sacramento  Hill  at  Blsbee  fox  example  oloarly  shows 
the  greater  resistance  of  pyrite  to  alteration  than  of  bornite. 
There,  it  will  be  recalled,  the  ratio  of  pyrite  to  chaloocite 
in  the  enriched  portions  ol  the  deposit  is  approximately  the 
same  as  the  ratio  of  pyrite  to  bornite  in  the  deeper  ores, 
which  indicates  clearly  that  the  pyrite  was  little  changed 
by  conditions  which  had  completely  altered  the  bornite* 
Under  the  microscope,  the  comparisons  are  based  on  the  vol- 
umes of  the  different  primary  minerals  which  have  been  re- 
placed by  chalcocite  or  its  associated  products  under  the 
same  conditions,  or  on  the  persistence  of  grains  of  one 
mineral  which  remain  in  fields  in  which  others  have  been 
partially  or  completely  replaced*  Examples  are  numerous  of 
veins  of  ch&lcooite  or  oovellite  in  bornite  pinching  or  dis- 
appearing when  the  crack  along  which  they  have  developed 
passes  into  chalcopyrite.   (Figure  155*)   Enarglte  frequently 
remains  unenriched  in  material  in  which  the  chalcopyrite  has 
been  attacked. 

Pyrite  itself  is  attacked  to  a  certain  extent, 
but  the  amount  of  chalcooite  which  ie  now  attributed  to  pyrite 
replacement,  ie  much  less  than  it  was  a  few  years  ago  when 
the  relations  and  distribution  of  the  primary  minerals  were 
less  thoroughly  understood*  The  later  minerals,  chalcopyrite 
and  bornite  particularly,  are  often  in  replacement  relation- 
ships to  the  pyrite,  either  breaking  it  in  veins  or,  where 


7 

341. 

more  abundantly  developed,  occurring  as  &  cementing  material 
about  the  grains.   (Figures  93  ,9h)«  Where  the  bornite  or 
ohaloopyrite  are  completely  replaced  by  chalcooite,  the 
same  forme  are  inherited,  and  in  the  resulting  product  the 
ohalcocite  appears  to  have  been  derived  by  the  replacement 
of  pyrite,  although  ae  a  matter  of  fact  this  mineral  may 
have  remained  completely  resistant  to  the  ohalcocite  attack. 

There  is  little  evidence  of  exact  comparative 
value  with  respect  to  tennantlte  and  tetrahedrite.  Under 
oonte  conditions,  they  are  corroded  by  the  ohalcocite,  but 
usually  they  remain  resistant  in  fields  in  which  surround- 
ing sulphides  are  partially  or  completely  altered. 

The  evidence  with  respect  to  sphalerite  and  galena 
Is  somewhat  conflicting.  According  to  Lindgren1,  the  altera- 
tion of  sphalerite  to  covollite  Is  the  deepest  change  which 
takes  place  at  Moreno  i,  but  this  statement  was  made  before 
the  reflecting  microscope  was  in  common  use  among  geologists 
and  hae  not  been  confirmed  by  the  evidence  it  has  yielded 
concerning  these  ores.  R.  U.  Over  beck2  states  that  sphalerite 
is  more  easily  replaced  by  chalcocite  than  is  bornite  but 
his  photographic  evidence  is  not  convincing.  For  the  most 


1.      The  copper  deposits  of  the  Clilton-Morenoi  District* 
Prof.    Paper,      3,   U.S.G.S.,    (1905). 


3  *     A  metallographio  Bt.udy  o.|.  ....thecop.  of  Maryland^ 

EC  on.   Geol.,    Vol.    II,   pp.   151-1787^1916)  ,  See   figure  9. 


-z 


42. 


part  sphalerite  is  slightly  more  resistant  to  tha  secondary 
sulphides  than  ohalcopyrite.  Galena  ia  not  very  commonly 
teen  in  relations  which  throw  definite  li-;ht  on  its  ease  of 
replacement,  but  on  the  whole  it  is  fairly  certain  that  it 
is  more  resistant  than  bornite.  There  is  apparently  no 
tendency  for  blebs  of  galena  in  partially  enriched  bornite 
to  be  aought  out  by  the  chaloooite,  while  on  the  other  hand 
ohaloocite  has  been  observed  associated  with  blebs  of  bornite 
in  areas  of  galena.  In  the  Marble  Bay  deposit  at  Texada 
Island,  there  is  evidence  that  galena  it  attacked  by  ohalco- 
oite  more  readily  than  chalcopyrite,  and  it  is  probable  that 
this  relation  is  the  general  one.  Intermediate  reactions 
between  those  described  for  pure  galena  and  those  described 
for  steinmannite1(the  arsenlc-actijaony  variety)  have  been 
observed.  It  is  reasonable  to  assume  that  the  rate  of  re- 
action with  copper  sulphate  solutions  would  also  vary,  which 
might  account  for  its  present  indefinite  position  in  the  se- 
quence. 

From  the  results  of  experiments  in  the  Geophysical 
Laboratory2,  it  was  found  that  the  sequence  of  minerals, 
bcb«ed  on  the  weight  of  material  altered  in  two  months  in 
1.25  per  cent  solution  of  oupric  sulphate  at  40°C,  is  as 
as  follows;  galena  (greatest)  (5.5),  bornite  (3.5),  ephaler- 

1.  J.  Murdoch,  Loo.  oit.,  Mineral  tables,  p.  133. 

3.  E.  G.  Zieeo,  E.  T.  Allen,  and  H.  E.  Herwin,   Loc.  cit. 


'Od 

o 

* 


i 


/  '.:  •  •  .: 


343. 


ite  (l.l),ohaloopyrite  (!•)•  'Based  on  volume,  the  series 
ie  : —  galena  (3.0),  bornite  (3.1),  sphalerite  (l.l),  ohalco- 
pyrite  (1.0).  In  precipitating  power,  bornite  follows 
ohaloopyrita,  but  in  volume  of  mineral  altered  it  follows 
galena.  As  the  microscopical  determination  of  the  ease  of 
replacement  is  based  on  volume  relations,  the  second  series 
is  more  significant  for  comparison  with  natural  relations* 
The  ease  of  replacement  of  bornite  is  well  shown,  but  the 
strong  reaction  in  the  oaae  of  galena  is  surprising.  How- 
ever, as  is  mentioned  by  the  authors  of  the  paper,  the  a- 
mount  of  material  dissolved  varies  notably  with  the  surface 
exposed,  and  it  is  very  probable  that  in  minerals  which  po- 
ssess a  good  cleavage  the  actual  surface  available  for  at- 
tack ie  greatly  increased  by  incipient  cracks  along  the 
cleavage  directions,  developed  during  the  grinding.  This 
factor  would  be  especially  important  in  both  galena  and 
sphalerite,  and  may  explain  their  higher  position  in  the 
series  established  by  chemical  work  than  observations  of 
associations  of  these  minerals  in  o^g-deposits  would  accord 
them. 

QrosQQpio  Relations. 

Knowledge  oi  the  microscopic  relations  between  the 
earlier  minerals  and  the  group  of  later  sulphides  is  always 
necessary  to  interpret  the  nature  of  changes  in  any  ore- 
deposit,  but  it  ie  of  especial  value  in  the  case  of  bornite 


3,4. 


ores,,  where  the  greater  depth  to  which  enrichment  can  ex- 
tend mokes  the  relations  observed  in  the  field  leae  definite 
ae  criteria,  and  throws  a  higher  relative  value  on  the  inter- 
pretation of  intimate  structures  observed  under  the  micro- 
scope. 

Simple  Structures*  Under  conditions  of  enrichment 
of  normal  intensity,  bornite,  aa  do  the  other  sulphides, 
alters  to  ohalcooite  along  velnlete  whose  course  is  clearly 
determined  by  preexisting  oracke,  or  along  grain  or  gangue 
boundaries,  where  definite  channel  ways  are  afforded  for 
the  enriching  solutions*   (see  figures  41,  and  7^). 
The  distribution  of  the  chalcocite  is  dependent  largely  on 
these  easily  recognised  causes,  and  usually  Dhows  little 
tendency  to  be  controlled  by  the  more  delicate  influences 
of  crystallographic  nature  or  other  internal  properties  of 
the  bornite. 

In  certain  deposits  in  ores  near  the  bottom  of 
the  enriched  zone,  the  chalcocite  works  its  way  around  the 
edges  of  the  individual  bornite  grains  within  the  massive 
material  which  results  in  a  peculiar  pattern  on  the  polished 
surface  of  large  and  snail  bornite  areas  apparently  floating 
in  ohalcoclte.  Where  the  bornite  grains  are  small,  replace- 
ment advances  more  rapidly,  consequently  a  matrix  of  finer 
grains  is  often  eaten  out  from  around  the  larger,  producing 
a  pattern  on  the  polished  surface  to  which  C.  F.  Tolman, 

1.  Loc.  cit.,  Fig.  13. 


/I 

345. 

has  given  the  descriptive  naae,  "ice-cake  structur-  • 

The  Lattice  Structure^  The  replacement  of  bornite, 
however,  under  certain  conditions  may  develop  other  structuree 
core  complicated  than  the  simple  ve inlets  or  rime  described 
above.   In  one  prominent  type,  which  ie  commonly  termed  the 
"lattice  struct  lire",  the  replacement  advances  in  a  differen- 
tial manner,  apparently  controlled  by  crystallographio  factors. 
Plates  of  ohaleocite,  covellite  or  chalcopyrite  develop  in 
tha  bornita,  parallel  to  two  or  three  structural  directions, 
and  when  sectioned  by  the  polished  surface  yield  a  reticulate 
grill  ol  intersecting  strips  ox  bands  in  a  field  of  bornlte. 
(:-"iCures  M  ,  7^  ,  77  ,13^,137  ,1^6  ,  and  152.)..  Chalcocite  is  the 
most  abundant  mineral  in  this  relation  with  bornite,  but 
oovellite  and  chaloopyrite  are  by  no  means  uncommon.  The 
structures  are  very  similar  in  the  three  cases,  and  indicate 

with  little  doubt  that  the  orientation  of  the  plates  is  due 

. 
to  the  common  hoot  mineral.  En&rgite  and  luzonite  (?)  have 

alno  br:en  observed  in  lattices  in  bornite,  but  they  are  un- 
common and  appear  to  be  of  primary  origin. 

On  the  polished  surface,  the  lines  of  the  replacing 
miner£i,l  in  the  lattice  structure  are  usually  oriented  para- 
llel to  three  directions  but  a  fourth  direction  ie  shown 
oocssionslly,  which  suggests  that  the  bornite  yields  to  the 
attacking  solutions  along  octahedral  planes.  The  octahedral 
cleavage  cracks,  developed  by  pressure  on  the  polished  sur- 
face, form  a  pattern  very  similar  to  the  lattice  structure, 


I 


/*- 

346. 


an  agreement  which  strong 3y  euggests  that  this  peculiar  re- 
placement structure  was  determined  by  the  bomite  cleavage* 
The  octahedral  form  of  both  the  lattice  structure  and  the 
pressure  cleavage  in  bornite  la  farther  suggested  by  their 
eimllarity  to  the  etch-structure  of  synthetic  isometric 
ohalcocite  which  ia  known  to  follow  the  octahedral  parting.^ 
(Figures  135  and  l4o.) 

The  replacement  origin  of  the  minerals  associated 
with  the  bornit©  in  the  lattice  structures  is  convincingly 
shown  in  most  of  the  deposits  st\>died,  but  since  other 
explanationa  have  been  advanced  by  two  writers  for  similar 
water ial  in  the  Butte  ores,  it  will  be  neoeesary  to  consider 
the  evidence  in  detail. 

Chalcopyrite,  when  developed  in  bornite  in  asso- 
ciation -?ith  ohalcooite  or  covellite  of  recognized  secondary 
origin,  nearly  always  assumes  the  lattice  fora  (fi-rureaW  , 
7? ,  .13^),  and  its  distribution  near  veinlete  or  in  the 
neighborhood  of  the  secondary  sulphides  clearly  indicates 
its  dependence  on  the  same  agencies  as  those  which  produced 
the  copper  sulphides.  There  can  be  little  doubt  that  the 
mirsral  is  &  common  product  of  the  alteration  of  bornite. 
Chalcopyrite  platee  in  bornite  similar  to  those  of  the  lat- 
tice structure  have  been  produced  artificially  in  the  Oeo- 
'hysioal  Laboratory2  by  treating  bornite  with  ouprio  sulphate 


2.  T.  Allen  and  H.  E.  Merwin,  Loc.  dit.,  p.  52- 

2.   E.  G.  Zies,  E.  T.  Allen,  and  H.  E.  Merwin,   Loc.  oit., 
.  479. 


-  . 


• 


347. 

solutions.  Chaloocite  is  the  chief  product  of  the  reaction 
but  within  the  bornite  grains  away  from  the  direct  attack  by 
the  reagents,  chalcopyrite  was  formed*   AB  chaloopyrite  Is 
alao  produced  by  the  action  of  sulphuric  acid  on  bornite  , 
its  formation  in  thie  experiment  and  in  nature  is  attributed 
by  the  members  of  the  Geophysical  Laboratory  staff  to  the 
action  of  sulphuric  acid  produced  by  the  alteration  of  bornite 

o 

to  chalcooite  or  covellite*   In  oases  where  the  development 
of  chaloopyrite  is  accompanied  by  shrinkage,  it  is  quite 
probable  that  the  alteration  was  accomplished  partly  or  wholly 
in  this  way  (figures^  and  lj-5),  but  where  no  volume  change 
can  be  observed,  which  is  the  commoner  case,  there  must  have 
been  a  distinct  addition  of  iron  to  the  space  originally 
occupied  by  the  bornite*  The  reaction  is  probably  more  com- 
plicated than  simple  acid  attack. 

From  the  evidence  in  the  ores  themselves  and  from 
the  synthetic  work  in  the  Geophysical  Laboratory  it  may  be 
regarded  as  definitely  established  that  chalcopyrite  associated 
with  bornite  in  the  lattice  structures  is  of  replacement  ori- 
gin, and  that  it  is  closely  associated  with  the  alteration 
of  bornite  to  chalcooite  and  oovellite,  a  change  which  or- 
dinarily tak-58  place  under  secondary  condition* » 


1.  Ibid.,  p.  476 

2.  Ibid.,  p.  480 


aid* 

svz 


6 


348. 

The  possibility  of  "reticulate  inter growth*"  of 
bornite  and  chaloopyrite  under  primary  conditions,  however, 
cannot  be  entirely  eliminated  for  somewhat  similar  structures 
were  formed  artificially  at  the  Geophysical  Laboratory  in  a 
sulphide  melt  from  which  products  close  in  composition  to 
bornite  and  ohaloopyrite  respectively  crystallized.  But, 
while  there  is  a  general  resemblance  in  a  broad  way  between 
the  structures,  the  details  are  distinctly  different,  and 
easily  distinguished.  Forms  exactly  similar  to  these  syn- 
thetic structures  have  never  been  observed  in  natural  prod- 
ucts. All  the  available  evidence,  however,  indicates  that 
the  chalcoclte,  oovellite  and  ohalcopyrite  in  lattice  struc- 
tures with  bornite  is  of  replacement  origin* 

The  common  distribution  of  oovellite  spines  around 
the  margins  or  grains  or  along  the  border  of  veinlets  in 
bornite,  often  closely  associated  with  chalcocite  or  prod- 
ucts of  oxidation,  makes  the  replacement  and  the  secondary 
origin  of  oovellite  in  this  relation  so  apparent  that  other 
interpretations  have  not  been  suggested. 

The  chalcocite  in  the  lattice  relation  with  bornite 
is  believed  to  be  of  replacement  origin  by  Graton  and  Mur- 
doch1 who  first  described  the  structure.  In  a  later  paper 
by  J.  C.  Ray2,  dealing  with  the  Butte  ores,  the  opinion  is 


1.  LOG.  cit. 

3.  The  paragenesia  of  the  ore  minerals  in  the  Butte  dis- 
trict, Montana,  Boon.  Geol.,  Vol.  9,  p.  479,   (1S14). 


349. 


expressed  that  the  bornite  in  lattice  structures  in  the 
"covellite  zone"  was  formed  by  the  elimination  of  iron  from 
the  chaloocite  along  crystallographic  planes.   He  states 
that  the  precipitation  of  ohalcocite  in  a  colloidal  state 
is  strongly  suggested  and  that  the  formation  of  chalcopyrite 
and  bornite  is  believed  to  be  due  to  a  reaction  of  iron 
locally  taken  into  solution.   In  a  discussion1  published 
more  recently,  however,  he  apparently  regards  the  chaloocite 
in  the  lattice  patterns  to  be  an  incomplete  replacement  of 
bornite,  and  his  excellent  photographs  strongly  support  this 
view.  Julius  Segall**  from  a  study  of  similar  material, 
interprets  the  relations  to  indicate  a  replacement  of  ohaloo- 
cite  by  bornite,  along  crystallographic  directions  of  the 
former  mineral.  W.  L.  Whitehead3,  however,  from  a  study  of 
material  from  many  localities,  supports  the  earlier  inter- 
pretation, advanced  by  Graton  and  Murdoch,  and  regards  the 
evidence  presented  by  Ray  and  Segall  to  be  unconvincing. 

In  all  the  ores  studies  in  the  course  of  this  work, 
there  has  been  distinct  evidence  that  the  chalcocite  in  the 
lattice  structures  is  closely  associated  with  chalcocite, 
which  from  its  occurrence  in  veins  or  rims,  is  plainly  of 
replacement  origin.  The  beginnings  of  the  lattice  patterns 

1.  J.  C.  Ray,  Econ.  Geol.,  Vol  11,   pp.  179-185,   (1916). 

3.      The  origin  and  occurrence  of  certain  crystallographic 
intergrowths,      Econ.    Geol.,    Vol.    10,     p.    467,      (1915;. 

3.     The  paragenesis  of  csrtain  sulphide  iatergrowthe, 
Econ.    Geol.,    Vol.11,     p.    10,      (1916). 


350. 

have  often  been  observed  as  fine  lines  penetrating  the  bornite 
from  the  edges  of  ordinary  veinlets  or  rims.  In  these  early 
stages,  the  stripe  of  chalcooite  are  most  readily  interpreted 
as  veinlets  in  the  bornite*  All  degrees  of  ohaloooite  abund- 
ance may  be  explained  aa  the  results  of  the  widening  and  ex- 
tension of  these  veinlete,  with  the  accompanying  gradual  re- 
duction and  elimination  of  the  ieolat-ed  rhombs  and  triangles 
of  bornite*  When  examined  under  high  magnification,  these 
residual  shapes  are  often  found  to  have  blurred  bluish  borders, 
due  to  a  fringe  of  tiny  bornite  specks  severed  but  not  yet 
destroyed  by  the  advance  of  the  ohalooclte.  The  blue  color 
in  these  transition  zones  may  be  due  in  part  to  exceedingly 
minute  bits  of  covellite,  which  is  known  to  form  as  an  inter- 
mediate product.  Hazy  boundaries  of  this  sort  are  common 
accompaniments  of  replacement  by  secondary  ohalcooite>  and 
these  relations  in  the  lattice  structure  prove  beyond  doubt 
that  the  ohalcocite  is  of  metasomatio  origin. 

While  the  views  advanced  by  Hay  and  Segall  may  seem 
to  explain  the  relations  observed  in  certain  special  oases, 
it  is  believed  that  the  abundant  evidence  of  replacement 
exhibited  in  many  districts  offers  a  more  generally  applic- 
able interpretation.  None  of  the  relations  described  by 
Ray  in  his  earlier  paper  presents  any  objection  to  the  re- 
placement origin  of  the  ohalcocite.  Segall 'e  arguments  are 
based  largely  on  the  resemblance  of  the  lattice  patterns  to 
the  triangular  etch-structure  of  ohalcooite,  and  on  the 


351. 


occurrence  of  bornlte  in  vein-like  bands  along  cracks  in 
ohaloocite  or  bornite-chalcocite  areas.  Chalcocite  v^ith  the 
triangular  type  of  etching,  however,  is  uncommon  except  where 
associated  with  bornite.  The  usual  orthorhombio  etch-pattern 
of  chalcocite  ie  not  similar  to  the  bornite-ohalcooite  lattice 
(Structure.  Moreover,  it  is  difficult  to  conceive  of  the  me- 
chanics of  a  replacement  process  which  would  develop  angular 
grains  of  the  new  mineral  between  regular  vein-like  strips 
of  other.  The  interpretation  would  demand  two  ages  of  bor- 
nite, for  the  evidence  of  the  later  age  of  some  of  the  chalco- 
cite is  undeniable  in  moat  deposits. 

Borders  of  bornite,  such  as  Segall  illustrates, 
alone  irregular  cracks  in  areas  of  ohalcocite  or  of  chaloo- 
cite  and  bornite  (figuresl't6andl 50), often  in  the  lattice 
relationships,  certainly  resemble  veinlets  of  bcrnite  cutting 
the  surrounding;  material.  As  a  matter  of  fact,  in  this  kind 
of  occurrence  as  well  as  in  related  structures  in  which  mar- 
ginal rims  of  bornite  surround  fields  of  mixed  ohalcocite 
and  bcrnite,  detailed  examination  under  high  magnification 
often  shows  that  the  bcrnlte  is  not  later  than  its  surround- 
ings, but  is  instead  residual  and  is  yielding) though  with 
apparent  resistance,  to  the  same  kind  of  replacement  by 
chalcocite  ae  that  which  day  be  shown  by  the  bornite  in  the 
general  field  of  replacement.1 


1.  For  an  excellent  example  eee  J.  Murdoch,  Op.  cit., 
Frontispiece!   Fig.  2. 


353 


Irregular  stripe  of  bornlte  in  fields  partially 
altered  to  chalcocite,  or  rime  of  bornite  about  areas  con- 
taining chalcooite  and  bornlte  have  b»en  observed,  which  on 
first  inspection  apparently  support  Segall's  interpretation, 
via.,  that  the  bornite  is  a  replacement  of  the  chalcocite. 
However,  detailed  examination  under  greater  magnification 
rarely  fails  to  reveal  fine  veinlets  of  chalcooite  or  other 
relations  indicating  partial  alteration  of  the  bornite  to  the 
ohalcocita,  which  make  it  highly  probable  that  the  elongated 
forme  of  the  bonnlte  are  inclusions  and  not  veins.  The  pos- 
sible explanation  may  be  suggested  that  this  peculiar  pres- 
ervation of  the  bornite  from  chalcocite  attack  was  due  to  the 
concentration  of  outgoing  iron  solutions  produced  by  the  re- 
placement reactions. 

The  differential  attack  of  the  ohaloooite  in  the 
lattice  structure  whereby  certain  strips  are  altered  and  others 
left  suggests  the  explanation  that  the  alteration  was  produced 
slowly  under  mild  conditions,  probably  by  dilute  solutions  act- 
ing for  long  periods  of  time.  Where  enrichment  is  intense,  the 
lattice  structure  is  rare;  it  is  commoner*  In  the  deeper  parts 
of  the  enrichment  zone  where  bornite  is  the  only  mineral 

I 

altered,  or  in  protected,  impermeable  cores.  This  distribution 
is  clearly  shown  at  Bisbee.  The  structure  is  well  known  in 
ohalcocite  ores  of  accepted  secondary  origin,  and  it  seems 
probable  that  the  milder  and  more  slowly  acting  processes  of 
seoondary  enrichment  would  be  more  favorable  for  its  production 


8i    ft' 


353. 

than  the  more  intense  action  of  heated  ascending  solutions  of 
the  period  of  primary  mineralization.   The  occurrence  of  a 
structure  of  auch  delicacy  under  two  sets  of  contrasted  con- 
ditions, while  possible,  would  be  rather  unusual,  and  the 
presence  of  the  lattice  or  oloeely  related  structures  in  the 
ores  at  Kenneoott  and  at  Butte  may  be  regarded  as  an  argument 
in  favor  of  ji  secondary  origin  for  the  ohaloooite  in  these 
disputed  occurrences. 

Residual  Structures  Related  to  the  Latti ce  Structure . 
Associated  with  the  lattice  structure  are  various  modifications, 
in  part  closely  related  in  form,  and  in  part  transitions  to 
other  structures.  Of  the  former,  the  most  prominent  is  the 
occurrence  of  definite,  sharply  bounded  plates  of  bornite  in 
the  chalcooite,  or  ohalcocite-bornite  lattice,  which  apparently 
remain  resistant,  to  the  replacing  reactions.   (Figures  1^3, 
1^7  andlW).  From  the  appearance  of  these  plates  on  the 
polished  surface,  the  name  'spine  structure"  has  been  commonly 
applied  to  it,  but  as  this  expression  is  not  accurately  descrip- 
tive for  the  rather  blunt  sections  of  the  bornite  plates,  as 
revealed  on  a  polished  surface,  the  term  spline  structure1  is 
suggested.  Where  associated  with  the  chalcocite-bornlte  lat- 
tice, the  plates  or  splinee  are  usually  oriented  symmetrically 
but  not  parallel  to  the  lattice  directions.  The  edges  against 
the  chalcocite  commonly  yield  evidence  of  replacement,  and 


1.  Spline,  a  thin  plate  in  a  slot,  a  term  used  in  con- 
nection -with  machinery  and  carpentry. 


• 


- 
. 

. 
c 

. 


354. 

/"  *t 

i 

where  the  enrichment  is  intense,  they  may  be  broken  by  numer- 
ous irregular  cross  veinlets  of  ohalcooite.   (Figure  143 . ) 
They  rarely  yield  a  lattice  pattern  by  their  decomposition, 
but  the  alteration  along  their  edges  is  often  controlled  by 
the  directions  of  the  adjoinia^  lattice.   (Flguresl^Tandl1^.* ) 
The  strips  of  bornite  isolated  between  long  bands  of  ohalco- 
oite  in  the  lattice  structure  are  rarely  as  long  as  the  splines 
described  above,  and  are  usually  broken  by  regular  cross-bands 
of  ohaloocite.  Their  contacts  with  the  chalcocite  are  of  the 
hazy  indefinite  type  for  the  most  part.  Consequently  there 
is  little  indication  that  splines  aro  normal  residues  of  this 
sort.  There  is  a  distinct  suggestion  that  the  bornite  in 
these  forms  is  of  different  character  from  that  which  filled 
the  surrounding  areas,  for  the  difference  in  resistance  to 
ohaloocite  replacement  is  very  marked  and  la  difficult  to  ex- 
plain on  other  grounds.  This  possibility  will  be  discussed 
more  fully  on  later  pages. 

Numerous,  sharply  defined  grains  of  bornite  are 
often  left  scattered  through  the  chalcocite  field,  after  the 
replacement  has  almost  obliterated  the  angular  bornite  resi- 
dues of  the  lattice  structure.  The  grains  are  usually  very 
small,  mere  dote  on  the  polished  surface,  but  by  their  true 
color  they  are  in  notable  contrast  to  the  indefinite  residues 
of  bluish  bornite  which  mark  the  last  stages  of  the  lattice 
replacement.  The  specks  are  usually  irregularly  scattered 
but  their  distribution  preserves  the  lattice  pattern  by  their 


. 


-It 


'a  eo. 


355. 


avoidance  of  the  white  etripa  of  chaloocite  which  mark  the 
lines  along  which  the  first  replacement  was  accomplished. 
In  eorae  of  the  Dutte  ores  in  which  this  spot  structure  is 
particularly  prominent,  (Fi-rurelV?),  the  arrangement  of  the 
grains  sometimes  shows  curving  banded  patterns  imposed  upon 
a  field  in  which  the  lattice  structure  ia  still  strongly 
evidence. 

The  spot  structures  are  most  prominently  developed 
in  the  Butte  and  Bisbee  bornite-chalcoclte  ores*  In  some 
oases,  where  the  grains  were  somewhat  larger  than  usual,  they 
assume  smoothly  irregular,  forma,  very  similar  to  blebs  of 
bornite  in  the  graphic  stiucture.1  While  the  most  typical 
graphic  structures  have  not  been  observed  in  this  association, 
where  the  surrounding  ch&lcocite  had  clearly  been  formed  by 
lattice  replacement  of  bornite,  the  approach  to  true  graphic 
forms  is  so  close  in  many  oases,  as  to  afford  a  strong  argu- 
ment that  the  graphic  and  lattice  structures  are  of  related 
origins.  As  in  the  case  of  the  splines,  the  existence  of  a 
more  resistant  type  of  bornite  is  suggested  by  the  presence 
of  these  sharply  defined  grains,  remaining  in  fields  in  which 
all  the  rest  of  the  bornite  has  been  nearly  or  completely 
altered. 

Chalcooite  Derived  from  Bornite. 
Chalcocite,  which  has  been  derived  from  bornite 

1.  See  photograph  by  J.  C.  Ray:  Econ.  Geol..  Vol.  11, 
p.  184,   Fig.  6,   1916. 


arf  •»•*  fa 


iei»  'v 

• 


'<W 


356. 


through  the  stages  of  a  lattice  replacement,  retains  a 
structure  parallel  to  the  lines  of  the  lattice,  which  is 
revealed  by  bands  and  strips  of  white  in  a  blue  field  on 
the  polished  surface,  or  by  the  triangular  etch- pattern 
when  treated  with  reagents.   (Figures  119,  107,  102,  and  1^2,) 
The  lines  of  white  chalcocite,  when  possible  to  trace  them 
into  bornite-chaloocite  lattice  areas,  are  the  lines  of  the 
initial  strips  which  penetrate  the  bornite  fields*  The 
disintegration  of  the  angular  bornite  residues  between  the 
lines  yields  a  bluish  typo  of  chaloooite.  The  hazy  bluish 
tints  about  the  bornite  margins  are  largely  due  to  minutely 
spaced,  almost  submicroscopic  specks  of  bornite,  isolated 
by  the  advancing  chalcocite,  but  in  some  oases  the  color  is 
probable  due  to  fine  shreds  of  covellite,  formed  with  the 
chalcocite*  The  blue  color  of  the  chalcocite  suggests  the 
presence  of  dissolved  ouprio  sulphide. 

Etched  with  dilute  nitric  acid  or  potassium  cyanide 
solution,  chalcocite  of  this  blue  and  white  type  usually 
develops  a  perfect  etch- structure,  with  three  or  four  sets 
of  equally  developed  cracks,  identical  in  orientation  with 
the  lines  of  the  lattice  structures.  Where  the  etch-patterns 
may  be  traced  into  a  bornite-ohaloooite  lattice,  the  etch- 
cracks  are  invariably  found  to  be  parallel  to  the  bands  of 
ohalcooite.  With  one  possible  exception  no  oases  have  been 

1.  Eugen  Pootijak,  E.  T.  Allen,  H.  E.  Merwin,  LOG.  oit. 


. 


i   &4 


357. 

observed  in  which  this  structure  occur a   in  chalcocite  that 
is  known  to  be  a  replacement  of  any  sulphide  other  than  bor- 
nite* 

The  orthorhomblc  form  of  chalcocite,   which  ia  stable 
below  91°  C, possesses  an  etch  structure  on  the  polished  sur- 
face which  may  be  easily  distinguished  from  the  isometric 
type  or  from  that   inherited  from  the  lattice  structure. 
Orthorhombic  chalcooite  possesses  a  strong  etch-cleavage 
parallel  to  the  base,   a  weaker  one  parallel  to  the  side  pina- 
coii,   and  an  imperfect  one  parallel  to  the  front  pinacoii.1 
On  the  polished  surface,   etching  usually  yields  one  set  of 
strong  parallel  cracks  with  or  without  a  second  series  of 
weaker  cracks  at  right  angles.     Chalcocite  formed  under 
secondary  conditions  would  possess  the  orthorhorabio  form, 
for  the  temperatures  of  such  processes  are  always  well  be- 
low 91°  C. 

However,    in  a  number  of  deposits  ohalcocite  of 
known  secondary  origin  yields  the  triangular  etch-pattern 
closely  similar  to  that   of  high  temperature  chalcocite,   but 
in  all  cases  thus  far  encountered  the  chalcocite  has  been 
found  to  be  a  replacement   of  bornite,   from  which   its  struc- 
ture was  probably  inherited,   presumably  through  the  lattice 
stage.     The  explanation  has  been  definitely  confirmed  in 
certain  ores. 

1.      Joseph  Murdoch,      Op.   cit.,    p.    107. 


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358. 


An  inheritance  of  cryetallographic  orientation  of 
one  mineral  by  another  ie  not  an  uncommon  phenomenon,  but 
the  inheritance  of  internal  crystal  structure  ie  undoubtedly 
rare.  It  is  perhaps  difficult  to  comprehend  the  inheritance 
of  that  particular  molecular  arrangement  which  determines 
cleavage,  but  it  is  conceivable  that  certain  planes  of  weak- 
ness, parallel  to  the  cleavage  directions,  which  directed 
the  advance  of  the  first  stripe  of  lattice  chalcocite,  could 
be  preserved  as  apparent  cleavages  or  pseudo-partings  in  the 
chalcocite  in  the  same  way  that  a  visible  crack  which  has 
localized  replacement  on  a  coarser  scale,  often  remains  as  a 
line  of  weakness  within  the  final  product.  This  property, 
which  is  neither  cleavage  nor  parting  merits  a  separate 
designation,  and  for  it  the  term  herifrage  (from  the  Latin 
heres,  heir  as  in  inheritance,  and  fragilis,  from  frango, 
break)  is  proposed  by  L.  C.  Qraton  and  the  writer.  It  may  be 
defined  as  the  property  of  breaking  or  etching  along  particu- 
lar planes  of  systematic  arrangement  acquired  by  replacement 
of  an  earlier  mineral  which  possessed  cleavage  in  correspond- 
ing directions. 

The  triangular  etch-structure  is  known  in  secondary 
ores  although  it  IB  not  a  very  common  feature,  but  it  cannot 
be  limited  to  chalcocite  of  secondary  origin.  It  is  most 
strikingly  developed  in  the  great  ohaloooite  ore-bodies  at 
Kenneoott,  for  which  a  primary  origin  is  favored.  There  is 
the  possibility  that  the  development  of  bornite  lattice  struc- 


/ 

*M. 

ture  and  of  the  related  triangular  pattern  in  chaloooite  is 
facilitated  at  temperatures  above  91°  C,  at  which  the  chalco- 
cite  possesses  an  isometric  form  similar  to  that  of  bornite, 
but  the  relationship  is  certainly  not  confined  to  such  con- 
ditions. Although  the  distinction  between  the  isometric 
structure  of  hi^h  temperature  chaloocite  and  the  structure 
inherited  from  bornite  cannot  always  be  made,  it  is  certain 
that  material  possessing  this  structure  is  either  of  primary 
nature  or,  if  secondary,  is  a  replacement  of  bornite.  Con- 
sequently, chaloocite  of  this  sort  would  either  be  expected 
to  continue  with  depth,  or  to  be  succeeded  by  primary  bor- 
nite, which  would  not  mean  a  very  serious  decrease  in  copper 
values . 


\ 


• 


360 


THE  GRAPHIC  STRUCTURE. 
Description 

The  tarn  "graphic  structure"  has  been  applied  to 
the  relation  between  bornite  and  various  other,  sulphides  in 
which  the  minerals  are  associated  in  a  manner  resembling  my- 
merkitic  or  micropegmatitic  intergrowths  of  quartz  and  feld- 
spar. The  structure  is  characterized  by  the  association  of 
the  two  minerals  (rarely  more)  in  minutely  sinuous  strips  of 
varying  widths  or  elongated  blebs  and  lobes  of  sub-equal  size. 
The  strips  are  simple  or  irregularly  branched  and  in  some 
oases  complexly  interdigitated.  A  rough  parallelism  in  one 
or  more  directions  is  not  uncommon,  and  in  some  instances  it 
approaches  a  regularity  sufficient  to  suggest  a  relation  to 
crystallography  directions*  The  contacts  between  the  two 
minerals  are  nearly  always  sharp,  and  rarely  offer  any  evi- 
dence indicating  relative  sequence.  As  many  tongues  of  bor- 
nite extend  into  the  associated  mineral,  and  as  many  projec- 
tions of  the  latter  are  apparently  severed  by  the  bornite  as 
the  reverse*  In  other  words  the  boundaries  of  the  two  miner- 
als are  mutual. 

These  associations  have  commonly  been  called 
"graphic  intergrowths",  but  since  the  word  "intergrowth" 
is  often  used  in  the  sense  of  simultaneous  formation  of  the 
two  components,  and  since  such  an  origin  cannot  be  univer- 
sally established,  a  term  that  is  merely  descriptive,  like 


361> 


graphic  structure   is  to  be  preferred,   for  it  implies  no  theory 
of  origin. 

The   following  minerals  have  been  observed  associated 
with  bornite   in  the  graphic  structure:--  ohalcopyrite,  galena, 
enargite,   tetrahedrite,   tennantite,  klaprotholite,   and  ohalco- 
cite.     Of  these,  the  bornite-ohalcooite  association  is  far 
more  abundant  and  of  much  greater   inter eet  than  the  others, 
but  as  it  will  be  treated  at  greater  length,   the  occurrence 
and  significance  of  the  less  common  minerals  will  be  dis- 
cussed first* 

Primary  Minerals  Associated  with  Bornltfi 
in  the  Graphic  Structure* 

The  ordinary  primary  structure  exhibited  between 
bornite     and  ohalcopyrite   is  not  similar  to  the  graphic  form 

7  (Figures  ^7  and  67  ),  but  in  a  few  oases,  as  at  the  Evergreen 
Mine  the  relations  between  the  two  minerals  are  practically 
those  ordinarily  observed  in  the  well-known  bornite-ohaloocite 

'&        graphic  pattern.      (Figures  ^5  and ^6.)       The  bornite  and  chalco- 
pyrite  are  generally  believed  to  be  essentially  contempora- 
neous, and  in  this  particular  case  there  is  every  reason  to 
believe  that  the  graphic  structure,  which  is  exhibited  be- 
tween them,   is  of  direct  primary  origin,  and  the  result  of 
the  simultaneous  deposition  of  the  two  minerals.     In  the 
neighborhood  of  the  graphic  structures,  however,  the  rather 
common  feature  of  slight  primary  corrosion  of  ohalcopyrite 
by  bornite  can  usually  be  seen,  which  is  regarded  as  an  indi- 


bev 


"0     ., 


Ixe  ai 


382. 


cation  that  the  initial  conditions  of  contemporaneous  de- 
position were  succeeded  by  those  under  which  ohalcopyrite 
yielded  a  little  to  bornite  replacement  t 

Bornite  and  galena  in  graphic  relations  are  more 
common*  The  distribution  of  the  two  minerals  indicates  that 
both  are  primary  in  all  examples  in  which  the  structure  has 
been  observed,  and  the  absence  of  all  evidence  of  replacement 
nature,  indicates  that  the  two  minerals  are  of  simultaneous 
deposition  and  that  the  structure  between  these  minerals  like- 
wise is  that  of  a  contemporaneous  intergrowth.  The  bornite 
usually  predominates  over  galena  and  is  the  host  mineral* 
The  bornite-galena  intergrowths  differ  from  the  usual  form  of 
the  graphic  structure  by  the  marked  variation  in  size  of  the 
galena  blebs  (figures  £9  and  90),  which  range  in  some  cases 
from  fairly  coarse  grains  down  to  the  finest  specks  that  can 
be  distinguished  under  the  oil-  immersion  lens*  An  intergrowth 

of  bornite  and  galena,  coarse  enough  to  be  visible  with  the 

1 
unaided  eye,  has  been  described  by  H.  P.  Collins. 

Tennantite  and  tetrahedrite  are  uncommon  in  the 
graphic  structure,  but  occur  occasionally  accompanying  other 
minerals  in  this  relation.  In  certain  fields  of  graphic 
structures  between  chalcocite  and  bornite  a  few  of  the  light 
colored  blebs  are  found  to  be  tetrahedrite  or  tennantite,  but 
in  the  same  shapes  and  relations  as  the  more  abundant  chalco- 

1.  Mineral  epical  Magazine  }  Vol.  13,  p.  336,   (1903). 


•jcevo  a«f?.; 

•-.:•  3  la'x 


. 


«?  (  .;« 


50  OC 
vT 


f 

363. 

cite.  A  similex  association  of  tetr&hedrite  and  tennantite 
with  galena-bornite  graphic  intergrowthe  has  been  noted* 
Graphic  blefcs  of  enargite  in  bornite  have  been  seen  in  a 
few  specimens  from  Butte.  An  unknown  pinkish  mineral  from 
Butte  also  shows  graphic  relations  to  the  bornite*  A  few 
grains  of  klaprotholite,  a  rare  eulphobismuthite  of  copper, 
which  has  been  doubtfully  identified  in  bornite  from  the 
Marble  Bay  Mine.  Texada  Island  in  connection  with  this  work 
occurs  in  forme  closely  simulating  the  bleb*  Of  the  graphic 
structure.  A.  F.  Rogers,   however,  has  described  klaprotho- 
tite  in  graphic  relation  with  bornite  in  ore*  from  Butte  and 
other  camps,  but  in  general  the  mineral  occurs  in  very  un- 
important quantities*  Rogers  states  that  it  is  later  than 
the  bornite. 

Chaloooite  in  the.  Graphic  Structure. 
The  minerals  in  the  graphic  relation  which  have 
been  described  in  the  last  few  paragraphs  are  of  primary 
origin,  and  with  the  possible  exception  Qf  klaprot hoi ite, 
are  probable  intergrown  simultaneously  with  the  bornite. 
The  origin  of  ohalcocite  in  the  graphic  structure  with  bor- 
nite is  less  easily  settled,  however,  and  offers  a  problem 
of  definite  commercial  as  well  as  scientific  interest.  The 
relations  upon  first  inspection  suggest  a  contemporaneous 

1.  Econ.  Geol.,  Vol.  17,  p.  584  (see  Fi£.  4),   1916. 


. 


ixow  sJMtf   rffrr. 


e^ 


awiiuc 
«iff     .  MHO  i 

.. 


r 

364. 


origin  for  the  ohaloooite  and  bornite,  and  henoe  that  the 
chalcooite  was  formed  under  primary  conditions,  The  abund- 
ant distribution  of  chalcocite  as  a  secondary  mineral  in 
most  camps,  however,  make 8  any  relation  which  may  indicate 
a  primary  origin  worth  especially  careful  atudy,  and  demands 
a  critical  examination  of  the  evidence  before  any  other  inter- 
pretation than  secondary  can  be  accepted* 

Bornite  and  chalcocite  are  occasionally  associated 
In  a  structure  identical  with  that  often  observed  between 
chaloopyrite  and  bornite.   (FigureslOlaJid  102*)  The  structure 
ie  similar  to  the  graphic  pattern  in  that  the  two  minerals 
exhibit  mutual  relations  for  the  most  part,  but  differs  in 
the  larger  size  of  the  blebs  and  in  their  often  pointed  and 
scalloped  forms*  The  two  structures  are  probably  of  about 
the  same  origin* 

Literature.  The  graphic  structure  between  bornite 
and  ohalcooite  was  first  described  by  F*  B.  Laney   in  ores 

from  the  Virgilina  District,  Virginia.   It  had  been  previously 

o 
suggested   that  the  deep  ohalcooite  ores  in  the  district 

were  of  primary  nature,  and  this  view  was  supported  by  Dr, 
Laney  who  regarded  the  graphic  structure  as  the  result  of 
a  primary  intergrowth  of  ohalcocite  with  bornite  .  Graton 


1.  F.  B.  Laneyt  Proc.  of  0.  S.  Nat.  Museum,  Vol.  40, 
p.  520j and  The  relation  of  bornite  and  chalcooite  in  the  cop- 
per ores  of  the  Virgilina  District  of  N.  C.  and  Va.,  Econ. 
Geol.,  Vol.  6,   pp.  399-411,   (1911). 

3.   L.  C.  Graton,  Mineral  Resources,  U.S.G.S.,  1907, 
Part  I,  p.  630. 


•SAX  8 


li       t» 

ei/oivs: 


• 


365. 

.nd  Murdoch   in  their  paper  on  the  sulphides  of  copper 
accepted  the  graphic  relation  as  an  indication  of  contempo- 
raneity, and  hence  a  criterion  for  the  primary  nature  of 
the  chalcooite.   The  same  interpretation  was  given  the 

M 

structure  by  C.  C.  Gilbert  and  G.  E.  Pogue   in  ores  from 

3 
the  North  lit.  Lyell  Mine  in  Tasmania*  R.  M.  Overbeck 

described  chaloocite  and  bornite  in  graphic  structure  in 
ores  from  Maryland,  and  believed  that  the  two  minerals  are 
contemporaneous.  It  is  suggested  in  the  description  of  a 
microphotograph  that  the  bornite  may  in  part  be  later  than 
the  chaloocite. 

Bornite  and  chalcocite  in  irregular  associations 
somewhat  akin  to  graphic  relations  were  described  by  A*  F. 

A 

Rogers   in  191*-,  and  believed  by  him  to  be  of  replacement 
character.  The  similarity  to  the  structures  described  by 
Laney  was  pointed  out,  and  it  was  suggested  that  the  same 
origin  may  be  applied  to  both.  No  proofs  were  advanced* 
In  an  earlier  article  on  the  Butte  ores,   the  same  writer 
came  to  the  conclusion  that  the  deep  chalcooite  at  Butte 


1.  L.  C.  Graton  and  Joseph  Murdoch,  loc.  cit.,  p*  768* 

2.  C.  C.  Gilbert  and  G.  E.  Pogue*  The  lit.  Lyell  Copper  dis- 
trict of  Tasmania*  Proc.  U.  S.  Hat.  Museum,  Vol.  45,  pp* 
609-625. 

3.  R.  M.  Overbeck,  A  met all ©graphic  study  of  the  copper 
ores  of  Maryland,  Econ.  Geol.,  Vol.  11,  pp.  151-178,  (1916); 
see  also,  B.  S.  Butler  and  H.  D.  kcCaekey,  Copper  ores  of  the 
Hew  London  Mine,  T.A.  liU.E.,  .  Tol.  49,  p.  287,   (1915). 

4.  A.  F.  Rogers,  Secondary  enrichment  of  copper  ores  with 
special  reference  to  microscopic  study,  Mining  and  Scientific 
Press,  Oct.  31,  1914,  p.  686. 

5.  Upward  Secondary  Enrichment  and  ohalcocite  formation  a* 
Butte,  Montana,   Vol.  8,  pp.  781-794,   (1913). 


366. 


Is   in  part  a  replacement  of  bcrnlte,  but  specific  reference 
was  not  made  to  the  graphic  structure*     Chaloocite  in 
graphic  structures   in  bornite  from  En^ele,  California  was 
described  by  A.   F.  Rogers,       and  believed  to  be  du*  to 
a  peculiar  break-down  of  bornite  into  ohalcoeite.     He  ex- 
preset i  th«  doubt  whether  graphic  relations  due  to  simul- 
taneous  intergrowth  ever  exist  between  these  minerals* 

2 
In  similar  material,  Ray,       ae  has  been  mentioned,   apparently 

regarded  both  the  bornite  and  chalcocite  as  replacements  of 
covellite,   but   considered  them  to  be   of  simultaneous  origin* 
Later       he  stated  that  the  graphic  structure  may  originate 
by  the  replacement  of  bornite  by  ohalcooite.     The  view  has 
been  advanced  by  L.   C.   Oraton4     for  graphic  structures  in 
general  that  some  may  be  simultaneous   intergrowths  and  others 
may  originate  by  replacement. 

The  replacement  origin  of  the  chalcooite  in  the 
graphic  structures  is  favored  by  W.   L*  Whitehead,     whose 
views  are  well  supported  by  hie  discussion  and  excellent 
mlcrophotographs .     A.   F.   Rogers,6     in  a  later  article,  comes 


1.     A*   F*   Rogers,     Engele  article,     Econ.   Geol.,     Vol.   9, 
p.   381.      (1914;. 

3.  J.  C.  Ray.  Econ.  Geol.,  Vol.  9,  pp.  463-487,   (1914). 

3.  Econ.  Geol.,  Vol.  11,  pp.  179-185,   (1916). 

4.  L.  C.  Graton,   Trans.  A.  I.  LI.  E.  ,  Vol.  58,  p.  597, 
(discussion). 

5.  W.  L.  Whltehead,  The  Paragenesis  of  Certain  Sulphide 
Intergrowthe.  Econ.  Geol.,  Vol.  11,  pp.  1-13, 

6.  A.  F.  Rogers,  The  so-called  graphic  intergrowth  of 
bornite  and  chaloocite,  Vol.  11,  pp.  587-5S-3,  Econ.  Geol., 
(1816,  ) 


— 

367. 


to  very  similar  conclusions  and  advances  some  additional 
arguments. 

It  ia  apparent  from  this  brief  survey  of  the 
literature,  that  there  is  notable  divergence  of  opinion 
concerning  the  origin  of  the  ohalcocite  in  the  bornite- 
chaloocite  graphic  structures*  The  evidence  in  most  oases 
admits  of  more  than  on*  interpretation,  and  it  is  difficult 
to  advance  arguments  of  sufficiently  compelling  nature  to 
settle  the  question  in  a  satisfactory  and  final  manner. 
Our  own  observations  confirm  the  relations  described  by  the 
other  workers  in  nearly  all  oases,  but  our  interpretation 
of  their  value  as  arguments  differs  somewhat.  The  evidence 
favoring  the  two  opposing  views  of  origin  will  be  summarised 
in  order  in  the  following  paragraphs,  and  on  the  later  pages, 
the  significance  of  the  various  relations  will  be  discussed. 

Summary  of  Evidence.  Evidence  favoring  a  contempo- 
raneous origin  lor  the  bornite  and  chaloocite  in  the  graphic 
structure  may  be  summarized  as  follows: 

(1)  the  similarity  of  the  graphic  structure  to 
the  structures  of  metallic  eutectics, 

(2)  to  the  structure  of  quarts  and  orthoclase 
in  micropegmatite;  and 

(3)  to  the  structures  between  bornite  and  primary 
ore-minerals; 

(4)  the  sharp  contacts  and  mutual  boundaries  be- 
tween the  bornite  and  oh-.lcocite; 

(5)  the  occurrence  of  the  otmlcooite  in  the  heart 
of  bornite  grains  with  no  visible  channel 
ways  for  the  entrance  of  altering  solutions; 


368, 


(6)  the  lack  of  relation  between  the  ohaloo- 
cite  and  grain  boundaries  and  gangue  contacts; 

(7)  the  contrast  to  the  structures  of  bornite 
and  chaloocite  in  specimens  in  which  the 
ohaloocite  is  definitely  known  tc  be  of 
secondary  origin; 

(3)  the  parallel  orientation  of  chaloooite  in 
the  blebs  over  considerable  areas* 


The  arguments  supporting  the  view  that  the  chalco- 
cite  is  a  partial  replacement  of  the  bornite  may  be  enumer- 
ated as  follows: 

(1)  the  occurrence  of  occasional  microscopic 
veinlets  of  chalcoclte  connecting  the  ohalco- 
cite  lobes; 

(2)  the  lack  of  discernible  boundaries  between 
the  chalcocite  of  the  veinlets  and  the 
chalcocits  of  the  lobes; 

(3)  the  common  cr yet allograph ic  orientation  of 
the  ohaloocite  in  each; 

(4)  the  gradation  and  merging  of  chalcocite  of 
replacement  origin  with  the  chaloocite  of 
the  bornite-chalcocite  graphic  structure; 

(5)  the  presence  of  small  spines  of  chalco- 
pyrite  in  the  bornite  blebs; 

(6)  the  replacement  by  chaloocite  of  blebs  of 
other  minerals  (ae  klaprotholite  or  galena) 
associated  in  graphic  relations  with  the 
bornite;  and 

(7)  the  presence  of  bornite  "residues"  arranged 
with  a  faint  suggestion  of  the  lattice  struc- 
ture in  ohalcocita  blebs  of  the  graphic  struc- 
ture. 

The  points  in  both  lists  will  be  discussed  in  the 
order  of  their  enumeration,  beginning  with  those  which  favor 
contemporaneous  origin  of  the  two  minerals. 


j*>£tjf 

•$,.»Ci    ¥•*£•  ^fflHBOfllq^l    ;  Si 

• 


rfci'v&Jbr'x/.  ad^ 
lo 


363. 


Arguments  Favoring  a  Contemporaneous  Origin. 
(l)  and  (2).   Although  the  similarity  between  the  graphic 
structure  and  metallic?  eutectics  is  striking,  the  evidence 
is  not  sufficient  either  from  our  knowledge  of  conditions  of 
ore-deposition  or  from  experimental  work  to  state  that  the 

graphic  structure  in  sulphides  is  of  eutectio  or  eutectoid 

1 

origin.  Whitehead   draws  attention  to  the  discordance  be- 
tween the  structure  of  metallic  eutectice  and  the  graphic 
structure,  shown  by  the  presence  of  an  excess  of  only  one 
constituent  in  the  former,  whil-->  in  the  latter,  both  con- 
stituents, bornite  and  chalcocite.are  commonly  present  in 
addition  to  the  association  of  the  two  in  the  graphic  areas. 
Relations  strictly  in  accord  with  artificial  products  formed 
in  a  closed  system  and  in  a  limited  time,  however,  should 
not  be  expected  under  the  changing  conditions  which  must 
have  obtained  during  the  formation  of  the  natural  sulphides. 
Both  quartz  and  feldspar  in  excess  are  common  accompaniments 
of  micropegmatite  yet  this  structure  is  believed  by  nearly 

all  observers  to  be  a.  simultaneous  intergpowth  ^f  the  two 

2 

minerals.  E.  S.  Bast  in   has  presented  evidence  which  indi- 
cates that  the  coarser  graphic  structures  in  pegmatites  are 
not  of  eutectic  nature,  but  he  believes  that  the  components 
are  of  contemporaneous  origin.  The  sulphides  may  likewise 

1.  LOG.  cit.  p.  5. 

2»  E.  S.  Baetin,   Origin  of  the  pegmatites  of  Maine, 
Jour.  Geol,   Vol.  18,   1910.   pp.  397-320.   Bull.  No.  445, 
U.S.G.P.,   1911. 


I0a  at 


i-: 


• 

l  a  to. 


^ 


• 


7    1; 


tXBC' 

sqoi 


-i 


Af\  is     AH 

•fc-V  *J       ~jJ 


. 


• 


570. 


be  simultaneous  Intergrowtha  even  though  not  euteotios,  al- 
though the  argument  baaed  on  eimilarity  of  pattern  is  slightly 
weakened  if  it  oannot  be  admitted  that  an  origin  similar  to 
the  metallic  euteotice  is  possible.  Further  relations  of 
the  graphic  structure  to  euteotice  are  discuseed  on  later 
pages. 

(3)  The  graphic  relations  between  chalcocite  and  bornite 
are  very  similar  to  th*  associations  between  bornite  and 
certain  primary  minerals  which  have  been  Described  on  pre- 
ceding pages*   For  the  most  part,  the  minerals  of  this  group 
are  unknown  or  uncommon  as  replacements  of  bornite,  and  in 
general  there  is  good  evidence  to  believe  that  they  are  of 
contemporaneous  origin  with  the  bornite* 

(4)  T7hile  sharp  contacts  are  common  features  of  sulphide*  in 
primary  relations  to  each  other,  their  existence  in  the  graphic 
structures  does  not  constitute  definite  evidence  of  contempo- 
raneous origin*  Examples  are  numerous  of  chalcocite,  clearly 
of  secondary  origin,  with  sharply  defined  boundaries  against 
bornite,  and  indications  are  accumulating  that  many  primary 
aeeooiations  of  minerals  with  clean-out  outlines  may  likewise 
be  a  result  of  replacement. 

(5)  The  occurrence  of  chalcocite  in  the  graphic  patterns  in 
the  heart  of  bornite  graina  has  now  been  observed  in  enough 
cases  to  greatly  reduce  the  possibility  that  ohalcooite  is 
related  to  definite  visible  channel  ways  which  are  unexpoeed 
by  the  section  revealed  on  the  polished  surface.   It  is  con- 


i 


& 


. 


->: 


to   e: 


}V 

371. 

oeivable  and  probable  that  material  can  be  transferred  by 
channel-ways  of  sub-microscopic  size,  or  by  diffusion  through 
the  massive  sulphide,  for  movement  of  this  sort  must  take 
place  in  the  ordinary  process  of  replacement  to  supply  new 
material  and  to  remove  waste  products  through  the  compact 
layer  deposited  in  the  earlier  stages  of  the  reaction.  The 
replacement  of  chaloopyrite  or  bornite  by  chalcooite  due  to 
descending  copper  solutions  affords  a  definite  example  of  this 
action  for  although  the  replacing  sulphide  adheres  firmly 
to  the  primary  mineral,  and  forms  an  apparently  impermeable 
coating,  the  alteration  often  goes  on  to  completion  in  spite 
of  it. 

Even  as Burning  this  mechanism,  no  explanation  is 
offered  for  the  frequent  lack  of  alteration  of  the  bornite 
in  the  outer  portions  of  the  graine,  which  are  normally  most 
easily  affected  by  the  replacement  processes.  Under  the 
usual  conditions,  there  is  no  known  reason  why  the  attacking 
solutions  should  have  withheld  action  until  the  centers  of 
the  grains  were  reached.  While  graphic  chalcocite  is  by  no 
means  confined  to  the  heart  of  grains  in  general,  the  occurr- 
ence of  many  definite  cases  in  this  relation  and  the  diffi- 
culty of  explaining  the  association  by  the  ordinary  mechan- 
ics of  replacement,  offers  a  strong  argument  againet  the 
view  that  the  structure  could  originate  by  the  metasomatic 
alteration  of  a  homogeneous  mass  of  bornite. 
(6)  and  (?)  The  increasing  number  of  observations  of  chalco- 


watt 


sriT      •  ••.ctiM 


t 

<K 


eirtt  :. 

^Xaill  aei 
•i&MiB'XAanJt  v  ,,-*£•  xacf  a*  n^   &. '  • 


372. 


cite  and  bornite  in  graphic  relation  from  deposits  of  various 
types,  indicates  that  the  association  of  the  two  minerals  in 
thic  form  possesses  many  properties  which  are  as  constant 
and  as  character  let  ic  as  those  common  to  sulphides  of  recog- 
nized replacement  origin.  The  distribution  of  chaloocite 
developed  by  metasomatic  processes,  generally  shows  a  marked 
control  by  the  crystalline  structure  or  the  material  replaced 
or  by  definite  and  obvious  channelways,  such  as  grain  sur- 
faces or  mineral  contacts  especially  those  with  gangue,  or 
cracks  penetrating  the  material  attacked.  It  is  equally 
characteristic  of  ohaloooite  in  the  graphic  structures  to  be 
independent  of  these  features.  Distribution  suggesting 
cryetallographio  control  i«  sometimes  teen  (Figure   ), 
but  in  general  no  guiding  influences  can  be  detected  which 
would  direct  the  advance  of  the  replacing  mineral  into  the 
forms  characteristic  of  the  graphic  pattern.  The  indifference 
of  the  ohaloooite  to  the  channelways  offered  by  gangue 
boundaries  is  a  very  common  feature  of  the  graphic  structure. 
It  is  well  shown  in  figures  £6,  g/,  and  &$  ,  in  oree  from 
the  Superior  Mine  near  Engels,  California.   In  both  examples 
the  line  of  attack  which  normal  replacement  reactions  would 
be  expected  to  follow  would  be  the  contacts  of  the  magnetite 
crystals  with  the  bornite.  There  ie  no  marked  preference 
ehown  by  the  ohalcooitw  for  this  position.  In  the  neigh- 
boring Engels  Mine,  where  there  is  a  notable  amount  of  second- 
ary sulphides,  the  contacts  of  the  gangue  minerals  and  eepe- 


•ioa 
io  o-UeJtte 


ir 
• 


f 

373. 


cially  the  chlorite  lathe  embedded  in  the  bornlte  were 
found  to  be  particularly  sought  by  the  replacing  ohalcocite. 
In  the  similar  oree  at  the  Superior  deposit,  however,  chlo- 
rite lathe  were  observed  included  in  bornite-chalcocite 
graphic  areas  with  apparently  no  influence  on  the  distribution 
of  the  ohalcocite.   (Figure  f*6J   It  should  be  mentioned, 
however,  in  this  particular  case,  that  the  quality  of  the 
argument  depends  on  the  age  assigned  to  the  chlorite.  The 
arguments  have  been  advanced  on  earlier  pages  (110-113) 
why  we  concluded  that  the  chlorite  is  earlier  than  the  bor- 
nite.  According  to  the  interpretation  ^?iven  by  A.  F.  Rogers, 
however,  the  cross-cutting  relations  in  this  and  other  oases, 
would  be  considered  the  result  of  the  replacement  of  bornite 
and  primary  chalcocit©  by  chlorite.  The  general  proposition, 
however,  that  ohalcocite  in  the  graphic  structures  shows 
little  dependence  on  gangue  boundaries  is  based  on  numerous 
observations  in  many  deposits,  and  does  not  rest  for  final 
proof  on  the  association  with  chlorite  or  sericite  alone* 
From  the  relations  discussed  under  this  and  the  earlier  head- 
ings, it  ie  fairly  certain  that  the  chalcocite  in  the  graphic 
structure  was  not  formed  by  replacing  solutions  attacking 
all  bornits  with  equal  vigor  along  lines  usually  selected  for 
such  alteration.  The  independence  of  g&ngue  bounda-riea  and 
other  ordinary  channelways  which  ie  exhibited  by  many  graphic 


1.   Eoon.  Geol.,  Vol.  9.,  pp.  359-391,   (1914). 

Econ.  Geol.,  Vol.  11,  pp.  131-128,   (1916 )%  See  Fig.  3. 


tac 


•  •^l^iVIIttXi    aLT'rir       no    A^n«Kn- 


•IXB»i-i 

• 


374. 


structures  is  a  atronpj  argument  against  the  replacement 
hypothesis  unless  some  oth*r  control  oan  be  shown  to  exist, 
as  would  be  afforded  by  a  variation  in  bornite  character  of 
such  nature  and  so  distributed  that  it  would  guide  the  altera- 
tion away  from  its  usual  loci  and  into  the  peculiar  forms  of 
the  graphic  pattern. 

(8)  The  parallel  orientation  of  the  ohalcoolte  in  the  various 
blebs  and  lobes  of  the  graphic  structures  is  clearly  brought 

out  by  the  etch-pattern.  This  character  of  the  graphic 

1 
chalcoolte  was  first  described  by  Laney,   who  regarded  it 

as  proof  that  the  chalcocite  was  a  skeletal  crystal  formed 
with  the  bornite  in  a  manner  very  similar  to  the  quartz 
in  a  quartz- feldspar  graphic  intergrowth.  This  explanation 
is  attractive  and  meets  with  few  objections*  but  its  force 
is  greatly  weakened  by  another  possibility.  It  is  known 
that  chalcocite  may  inherit  its  orystallographio  orientation 
from  the  bornite  which  It  has  replaced.  Consequently,  if  of 
replacement  origin,  the  common  orystallographic  orientation 
of  the  various  blebs  is  easily  explained  as  an  inheritance 
from  bornite.  The  etch- structure  of  the  bornite  in  the  blebs 
of  the  graphic  structure  usually  shows  that  the  bornits  is 
also  in  parallel  orientation  throughout ,  and  sometimes  for 
a  little  distance  into  the  surrounding  field*  The  strong 
direction  of  the  etch-pattern  of  th*  bornite,  however,  is 

1.  Loc.  cit. 


- 


•'•ria  \d  £oa£>}tee>ir 

rf  £1 


*t  adf 

a   Silt 


375. 


not  necessarily  parallel  to  the  strong  direction  of  the 
chaloocite  cleavage. 

With  these  mod ifioat ions  it  is  apparent  that  the 
parallel  orientation  of  the  chaloocite  in  the  graphic  struc- 
tures possesses  little  value  as  an  argument  favoring  con- 
temporaneous origin  for  the  two  minerals. 

This  exhausts  the  arguments  which  have  been  advanced 
in  favor  of  the  contemporaneous  origin  of  the  ohalcocite  and 
bornite,  and  on  a  whole  it  ie  apparent  that  they  present  a 
strong  case  against  the  view  that  the  graphic  structures  could 
originate  by  the  replacement  of  massive,  homogeneous  bornite* 
They  make  it  clear  that  many  properties  of  the  graphic  struc- 
ture are  closely  related  to  those  of  contemporaneous  inter- 
growthe,  but  nevertheless  the  evidence  does  not  show  with 
certainty  that  the  chalcooite  in  the  graphic  patterns  is  of 
contemporaneous  origin  with  the  bornite.  Additional  considera- 
tions are  presented  on  later  pages. 

Diaouseion  of  Arguments  Favoring  a  Rep^ace.ment  Origin. 
The  arguments  of  the  second  group,  viz.,  those  favoring  a 
replacement  origin  for  the  chslcocite,  are  discussed  in  order 
below. 

(1)  Whitehead   states  that  the  graphic  structure  originates 
by  the  replacement  of  bornite  by  cbalcocite  along  minute  inter- 
lacing veinl  te.   The  characteristic  pattern  la  attributed 

1.  Loo.  cit.,  pp.  6-9,  Figures  6,  7  and  8. 


ll 

376. 


to  a  stage  of  the  metasomatism   in  which  these  veinlats 
are  broadened  in  places  and  the  bornlte  masses  rounded 
where  the  ve inlets  intersect.  In  many  graphic  areas,  how- 
ever, even  the  fine  veinlete  described  by  Whitehead  cannot 
be  observed  under  the  oil-imnersion  lens,  and  from  our  ex- 
perience, it  would  seem  that  although  they  are  not  an  un- 
common accompaniment  of  the  graphic  structure,  they  are  not 
invariably  present.  If  the  graphic  structure  originated  in 
the  manner  suggested  by  Whitehead,  it  is  difficult  to  under- 
stand why  the  degree  of  replacement  in  these  structures, 
now  known  from  many  widely  separated  districts,  should  be 
so  similar.   Intersecting  lace-works  of  veinlete  have  been 
observed  in  bornite  in  which  no  tendency  toward  the  graphic 
structure  could  be  detected,  but  between  this  stage  and  the 
typical  graphic  structure,  no  relations  have  been  noted  which 
could  be  intermediate  steps  in  the  change.  No  explanation 
Is  offered  by  this  plan  of  origin  for  the  differential  charac- 
ter of  the  replacement.  Chaloocite  developed  in  this  way 
would  not  be  expected  to  differ  so  distinctly  in  distribution 
from  replacement  chalcocite  in  general,  consequently  the  ob- 
jections raised  by  the  points  discussed  under  preceding  bead- 
ings  remain  in  force.  Replacement  under  theee  conditions 
would  be  expected,  as  is  stated,  to  yield  rounded  bornite 
blebs  or  lobes,  but  not  rounied  chalcocite  masses.  The  mutual 
relations  between  th$  two  minerals  offer  serious  objections 
to  this  theory  of  origin. 


j 


c 


(2)  Etching  shows  that  chulcooite  of  veinlete  breaking 
bornite  blebs  does  not  continue  across  Intervening  chaloocite 
fields*  In  some  oases,   where  a  crack  was  observed  to  con- 
tinue, it  seems  very  probable  that  the  chalcooite  along 
such  channels  is  later  than  the  chalcooite  of  the  graphic 
areas,  yet  there  is  usually  no  definite  boundary  between  the 
two  chalcocites  at  the  main  ohalcocite-bornite  margin.  The 
ve  inlets  are  usually  very  narrow,  consequently  the  line  of 
contact  between  the  two  chalcocites  on  the  polished  surface 
would  be  very  short,  and  a  break  in  continuity  might  not  be 
visible*  A  physical  boundary,  emphasized  perhaps  by  a 
variation  in  color,  might  be  expected,  if  the  chalcocite  were 
of  two  ages,  but  its  absence  does  not  afford  a  convincing 
argument  in  favor  of  the  hypothesis  that  the  ohaloocite  in 
the  graphic  structure  is  of  the  same  age  as  that  in  the 

ve  inlets. 

(3)  The  directions  of  the  etch-pattern  of  the  ohalcocite  in 
the  small  ve  inlets  are  difficult  to  determine,   and  from  our 
Orfn  work,   there  is  no  evidence  to  indicate  that  the  two 
chalcooites  are   in  parallel  or  yst  allograph!  c  orientation. 
From  ^hitehead*a  statement,  however,  that  the  chaloocite  of 
the  veinlete  is  continuous  with  that   of  the  blebs  it   is  to  be 
inferred  that  h?  believed  tha  chaloooite  of  the  veinlets  to  be  in 
orystallographic  orientation  with  that  of  the  blebs.     But, 


1.     Superior  Mine,  Engela,  California  for  example,     p, 


"jrl  ai 


•atyixa 


378. 


even  if  this  Is  established,  It  ie  not  a  conclusive  argument 
against  the  later  age  of  the  velnlets  for  the  orientation  of 
the  ohaloocite  in  them  coull  conceivably  be  determined  by 
that  of  the  pre-existing  chaloooite,  just  as  the  orientation 
of  the  later  quartz  in  a  quartzlte  is  determined  by  that  of 
the  quartz  in  the  original  sand-grains* 

(4)  In  addition  to  the  fine  veinlets  discussed  above,  it  is 
very  common  to  find  chaloooite  of  the  graphic  structures 
passing  without  detectable  break  into  ohalcocite  which  is 
olearly  of  replacement  origin.   (Figures  70,  75  »  *nd  S2,)1 
Bands  of  chalcoolte  forming  alteration  rims  about  bornite 
areas,  widen  into  patches  of  chalcooite  interwoven  with 
bornite  in  th*  graphic  pattern.  Furthermore,  fine  veins 
or  tongues  of  ch&lcocite  in  some  oases  penetrate  the  sharp 
walle  of  the  graphic  bornite  blebs  or  the  surrounding  mas- 
sive bornite.   (Figure.  #2.)  Blue  hazy  boundaries,  of  the 
kind  already  described,  are  sometimes  found  between  bornite 
and  ohaloooite  in  the  graphic  association,  and  show  beyond 
question  that  the  replacement  conditions  had  been  active  to 
some  extent  at  least,  though  in  parts  of  the  same  graphic 
areas,  however,  these  hazy  boundaries  may  be  entirely  want- 
ing. 

These  various  relations  oan  of  course  be  explained 
as  the  effect  of  later  secondary  ohaloocite  imposed  on  earli- 


1.  A  photograph  by  A.  F.  Rogers  in  Econ.  Oeol.,  Vol.  11, 
(1916),  p.  588,  Fig.  6,  illustrate^  this  point  well. 


. 


379. 


er  material  deposited  contemporaneously  with  the  bornite. 
It  is  possible  that  lat*r  ohalcocite  would  ao  weld  itself 
to  the  earlier  that  the  boundary  would  show  no  break  in 
color,  cryetallographic  orientation  or  other  properties,  but 
it  is  more  difficult  to  accept  this  explanation  in  these  cases 
than  in  those  previously  discussed  where  only  fine  veinlete 
are  concerned*  Except  for  the  graphic  relation  itself,  there 
is  often  no  reason  to  assign  to  the  ohalcocite  of  these  struc- 
tures a  different  origin  from  that  of  adjoining  material  which 
without  question  has  been  formed  by  replacement  of  bornite. 

On  the  whole ,  the  relations  between  the  graphic 
chalcoclte  and  material  of  replacement  origin  are  in  many 
oases  so  intimate  and  so  general,  that  they  constitute  a  very 
serious  argument  against  the  universal  application  of  the 
hypothesis  of  strictly  contemporaneous  deposition  for  the 
two  minerals  in  the  graphic  structure.  The  strength  of  this 
objection,  however,  ia  very  greatly  reduced  by  a  modification 
of  the  hypothesis  which  will  be  presented  later. 
(5)  The  formation  of  chalcopyrita  as  a  product  of  the  altera- 
tion of  bornite  to  chalcocite  has  been  commonly  observed  in 
material  from  many  districts,  as  has  been  mentioned  previously, 

* 

and  consequently  the  occurrence  of  chaloopyrite  spines  in  the 
bornita  of  the  graphic  areas  suggests  a  replacement  origin 
for  the  associated  chalcocite.  The  argument  is  weakened, 
however,  by  the  fact  that  chaloopyrite  ie  not  a  common  ac- 
companiment of  the  graphic  structure,  and  that  there  is  the 


380. 


possibility  that  it  may  be  the  product  of  neighboring  re- 
placement reactions,  unasaooiated  with  the  chalcocite  of  the 
graphic  areas,  for  the  formation  of  secondary  chalcopyrite 
at  any  point  commonly  precedes  the  formation  of  chalcocite, 
and  may  take  place  some  distance  ahead  of  the  first  traoea 
of  enrichment.  On  account  of  these  reasons  the  occurrence 
of  occasional  opines  of  chalcopyrite  in  the  graphic  struc- 
tures does  not  offer  positive  support  for  the  hypothesis  of 
replacement  origin  for  the  graphic  structure. 
(6)  It  Is  conceivable  and  easily  granted  that  chalcocite 
associated  with  bornite  in  the  graphic  structure  could  have 
originated  by  the  replacement  of  an  earlier  mineral  similar- 
ly associated  with  the  bornite*  The  other  minerals  which 

have  been  observed  in  this  association  with  the  bornite, 

. 
however,  are  in  general  lees  susceptible  to  replacement  by 

ohalcooite  than  is  the  bornite  itself  and  as  a  matter  of  fact, 
with  the  exceptions  noted  below,  no  evidence  whatever  to 
support  such  a  view  has  been  observed*  Galena  may  be  an  ex- 
ception but  there  are  certain  peculiar  features  commonly  ex- 
hibited by  the  galena-bornite  graphic  structures  which  are  not 
found  in  the  chalcocite  patterns,  and  which  make  it  probable 
that  the  chalcooite  was  not  derived  from  this  mineral*  A.  F. 
Rogers1  has  described  specimens  from  several  districts  in 
which  klaprotholite  and  chalcocite  are  associated  in  the  same 

1.  Econ.  Geol.,  Vol.  11,  pp.  583-593,   (1916). 


381. 


graphic  area  with  bornite,  and  he  believes  that  the  chalco- 
oite  was  formed  by  the  replacement  of  the  klaprothollte  blebs, 
which  in  turn  had  previouely  replaced  bornite.  While  this 
interpretation  ie  a  possible  one,  and  may  be  sufficient  in 
certain  special  cases,  the  rareness  of  the  mineral  and  the 
absence  of  partial  stages  of  its  replacement,  makes  it  ex- 
tremely unlikely  that  it  of fere  an  explanation  which  ie 
generally  applicable.  It  should  be  noted  that  even  if  this 
hypothesis  is  accepted,  the  explanation  is  only  postponed 
one  step,  for  it  is  still  difficult  to  understand  how 
klaprothollte  replaces  bornite  in  these  peculiar  forms. 
(7)  In  material  from  Engels  and  from  Bisbee,  fine  stripe 
of  bornite  were  observed  in  chaloooite  between  bornite  blebs 
similar  to  those  of  the  graphic  structure.  The  arrangement 
and  association  of  the  bornite  strips  suggests  that  they  are 
residues  from  a  replacement  of  the  lattice  type.   If  this  is 
the  case,  the  replacement  terminated  against  the  lobns  of 
graphic  bornite  with  a  sharp  even  boundary  as  if  against  a 
mineral  of  great  resistance  to  alteration.  Lattice  struc- 
tures sharply  limited  to  certain  areas  have  been  described 
on  earlier  pages.  Usually  the  evidence  suggests  that  under 
certain  conditions,  chalcoolte  in  the  lattice  pattern  can 
develop  uniformly  in  all  parts  of  certain  grains  without 
extending  in  the  slightest  to  neighboring  grains,  but  only 
in  the  few  cases  in  the  two  districts  mentioned  have  re- 
lations been  observed  which  definitely  suggest  that  the  grains 


;s~ 


*»• 


subject  to  this  peculiar  type  of  metasomatism  may  be  of 
graphic  outline.  The  apote  and  specks  of  apparently  more 
resistant  bornite  which  remain  scattered  through  the  field 
in  certain  Butte  and  Bisbee  ores,  after  the  main  mass  of  the 

» 

bornite  has  been  awept  away  by  lattice  replacement,  sometimes 
occur  in  twisted,  curved  blebs  suggesting  graphic  forms. 
Since  they  are  clearly  residues  in  chalcooite  of  replacement 
origin,  their  resemblance  brings  support  for  the  view  that 
the  chalcocite  in  graphic  areas  originated  in  this  way. 
If  this  were  the  case,  however,  it  would  seem  that  partial 
stages  should 'be  commonly  found,  and  the  practical  lack  of 
these  is  a  strong  objection  to  this  genetic  mechanism* 

From  a  review  of  the  arguments  on  both  sides  of 
the  controversy,  it  is  apparent  that  neither  view  can  be 
established  without  serious  modification.  The  hypothesis  of 
contemporaneous  origin  fails  to  explain  important  relations 
in  many  examples,  and  the  hypothesis  of  simple  replacement 
origin  ia  also  inadequate  to  give  a  complete  solution.  The 
evidence  ie  so  conflicting  that  neither  extreme  view  can  be 
established  to  the  exclusion  of  the  other,  and  it  indicates 
the  difficulty  of  explaining  all  features  associated  with  the 
graphic  structures  by  a  single  kind  of  mechanism. 

Hypothesis  of  Contemporaneous  Deposition  Modified 

by  Replacement.   The  most  serious  objection  to  the  hypothec  is 
of  strictly  contemporaneous  origin  is  the  close  association 

often  observed  between  ohalcocite  in  the  graphic  areas  and 


. 

• 

• 

• 
iei 

.^•a 

. 

. 


- 


'  a  «  vrf   < 


383. 


chalcocite  of  replacement  origin.   A  modified  form  of  the 
hypothesis,  however,  reconciles  the  evidence  if  it  ia  assumed 
that  although  the  chalcocite  was  initially  deposited  con- 
temporaneously with  the  bornite,  it  continued  to  form  after 
the  bornite  had  become  unstable,  which  resulted  in  the  de- 
velopment oi  replacement  relations.   The  hypothesis  thus 
modified  is  more  in  agreement  with  the  usual  relations 
observed  between  primary  minerals  than  is  the  view  of  strictly 
contemporaneous  deposition.  The  relations  between  the  bor- 
nite and  chalcocite  would  be  very  similar  to  those  believed 
to  exist  between  chalcopyrite  and  bornite  in  many  deposits. 
The  sequence  of  th-  minerals  implied  by  thie  hypothesis  is 
expressed  graphically  in  Fig.  40  . 


Increasinq    amount    of 
minerals 


,te     or 


Fig.    40 


fta      lenotfi 

of        time,    since 
initial     deposition 


40^  t  Di-^ran:.  of  Sequence  of  Bornite  and  Chalcocite 

According  to  the  Pria-ry  Theory  for  th-  Chalcocite 

in  Graphic  Structures  t 


_ 

-.'js   si   it   \i 

• 

.... 

- 

• 
£e. 

' 


•y»X»        wvtf ... 


A 


:>\t 


• 


384. 


Tha  hypothesis  in  this  form  assumes  that  with  the 
changing  conditions  of  the  period  of  primary  mineralization, 
a  point  was  reached  at  which  ohaloooite  commenced  to  deposit 
with  the  bornite,  but  with  progressive  changes  in  concentra- 
tion of  the  solutions  (probably  a  gradual  reduction  in  the 
iron-copper  ratio),  bornite  ceased  to  form,  became  unstable, 
and  tended  to  alter  to  ohalcooite.   It  is  conceivable  that 
the  contemporaneous  deposition  of  the  two  minerals  was  of 
temporary  euteotic  character* of  which  the  graphic  form  is 
the  expression,  but  that  these  conditions  were  generally 
interrupted  more  or  leas  promptly  by  the  addition  of  compo- 
nents favoring  the  production  of  chalcooite,  which  thus 
brought  about  the  super-imposition  of  replacement  phenomena 
upon  the  euteotio  pattern.  By  this  hypothesis,  graphic 
structures  which  are  unassooiated  with  any  evidence  of  re- 
placement are  explained  equally  as  well  as  the  types  in 
which  replacement  is  apparent.  The  hypothesis  harmonizes 
the  evidence  which  favors  a  contemporaneous  origin  for  the 
two  minerals,  with  the  strong  evidence  which  indicates  that 
part  of  the  sane  chaloooite  is  often  of  replacement  origin. 

As  already  pointed  out,  the  occurrence  of  graphic 
structure  among  certain  pairs  of  sulphide  minerals  which 
rarely  if  ever  exhibit  replacement  relations  toward  each 
other  makes  it  reasonable  to  conclude  that  they  are  contempo- 
raneous intergrowths,  especially  as  we  know  that  simultaneous 
graphic  structures  can  be  produced  as  final  crystallizations 


•xa  01. 

.  .'  iti«q 

,lrfW    91 


•%   CXfcWOw  .  ,.£'J-i '.{£?'.£     ,"• 

0    •TjB    YE 


385. 


in  alloys.  Among  tnese  natural  intergrowths  the  mineral 
ooirjnon  to  moat  of  them  is  boruite,  from  which,  because  of 
ita  sequence  position,  the  inference  nay  be  drawn  that 
graphic  structures  are  most  favored  in  the  late  stages  of 
mineralization.  This  deduction  may  render  more  acceptable 
tha  idea  that  the  graphic  structures  between  bornite  and 
chaloooite  are  actual  intergrowths,  since  primary  ohaloooite 
if  it  exists  at  all,  is  in  part  later  and  probably  just  later 
than  bornite.  No  certain  weight,  however,  can  be  attached 
to  this  speculation. 

If  the  interpretation  is  correct,  it  of  course 
means  that  part  of  the  ohalcooite  in  replacement  relations 
with  bornite  may  be  of  primary  origin,  a  fact  which  makes 
chaloooite  the  last  member  of  the  sequence  of  primary  min- 
erals* The  sequence,  in  its  simplest  terms,  would  be  mag- 
netite or  pyrite  —  chalcopyrite  —  bornite  —  chaloocite. 
The  first  three  members  whose  positions  are  definitely  es- 
tablished, are  in  agreement  with  the  sequence  if  arranged 
according  to  decreasing  iron  suvl  increasing  copper;  hence 
on  chemical  grounds  chaloooite  is  the  logical  end  member. 

Although  chalcooite  is  definitely  known  to  be  of 
secondary  origin  in  most  of  the  important  copper  deposits, 
and  hence  formed  at  ordinary  temperatures,  synthetic  results 
indicate  that  it  is  stable  at  higher  temperatures,  and  may 
be  formed  under  the  usual  conditions  which  are  believed 
to  have  prevailed  during  the  deposition  of  primary  sul- 


t« 


JD0*   *. 

. 


. 


atUtAOi 


phides.   In  the  Geophyeioal  Laboratory,  ohalooolte  crystals 
have  been  formed  in  the  dry  way  at  a  temperature  above  350°C, 
and  in  the  wet  way  at  temperatures  as  high  as  350°C.   In- 
crease of  temperature  is  also  known  to  aooelerate  the  rates 

r^ 

a 

of  the  reactions  by  which  secondary  ohaloooite  is  formed. 

In  general  chuloooito  in  the  graphic  patterns 

occurs  most  prominently  In  the  deeper  iarts  of  bornite- 

3 
bearing  deposits.   Nearer  the  surface  the  structure  is 

usually  obliterated  by  the  more  complete  replacement  of  the 
bornite  by  chalcocite,  and  evidence  of  its  former  existence 
is  largely  removed.  For  the  most  part,  the  field  relations 
indicate  that  the  ohaloooite  in  the  graphic  structures  at 
any  definite  place  in  the  ore  was  formed  earlier  than  the 
period  of  intense  development  of  secondary  ohaloooite. 

The  evidence  of  primary  origin  offered  by  the  deep 
ohalcocite,  while  very  suggestive*  cannot  be  regarded  as  fi- 
nal, however,  for  the  reason  that  in  none  of  the  cases  now 
known  are  the  workings  deep  enough  to  be  beyond  the  influence 
of  descending  solutions,  and  beyond  the  point  at  which  see- 
on  l&ry  ohaloooite  has  developed  from  bornite, with  a  few  ex- 
ceptions mentioned  later  4&*4  whose  character  is  not  fully 

determined^,    fchft     ••eg--" -Jr^y    *.1t.*r.-it*"'       -f    hi-ir  vi  fc«    f.vf       A<. 


1.  Eugen     Posnjak,    E.   T.   Allen,   and  H.  E.   Kerwin,      loo. 
oit .,  pp.    5*0-521. 

2.  E.    G.    Zeis,   E.    T.   Allen,   and  H.   E.   lierwin,      loo.    oit 

3.  Butte,   Eiigeis,    Virgilina,   for  examples. 


e. 
,0  02o   wrotfja  •tyj 

-al 


'4        1.   •»        YW 


i 

. 
«i  »'i 


aao. 


' 

?oe»  n  -^cf 


sto 


dflittO 


. 
•  Jbi  -ysi4i  ai 


-II  *£  Le?. 

:  ess   i 


« 


-Xft 


Jon  el  ic 
^»B   M. 


, 


Virgil ina  District,    tiie  dines  are  not  ever  450  ft.  deep, 
and  although  thers  is  very  definite  evidence  that  the  rooks 
are  extremely  tight,  the  long  continued  exposure  to  erosion 
cakes  it  surprising  that  the  bornite  ores  are  not  core  com- 
pletely enriched. 

.  If  the  hypothesis  of  primary  ohaloooite  is  accepted, 
it  becomes  necessary  to  attempt  to  distinguish  between  pri- 
mary ohaloooite  of  repl&centent  origin  and  secondary  ohaloo- 
oite in  the  same  relatiouu.  At  the  Superior  k'ine  near 

Engels,  and  in  the  Kuskulana  District,  Alaska,  the  secondary 

and  climatic 
ohalcocite  formed  under  recent  physiographic  conditions  is 

A 
so  feeble  that  it  is  in  definite  contrast  to  the  chaloooite 

in  the  earlier  graphic  or  related  structures.  But  in  cost 
oases  where  the  secondary  sulphides  are  well  developed  under 
present  conditions,  ohalcooite  definitely  recognized  as  sec- 
ondary in  origin,  may  be  traced  without  change  into  chaloo- 
oite of  replacement  origin  intimately  associated  with  the 
graphic  areas.  No  break  in  structure  or  other  properties 
can  be  detected,  and  any  limit  to  the  extent  of  the  sec- 
ondary ohaloooite  is  quite  arbitrary. 

While  this  is  obviously  a  difficulty  in  the  way 
of  the  acceptance  of  the  hypothesis,  the  possibility  of 
chaloooite  assuming  similar  replacement  forms  and  similar 
phyeioal  properties  under  primary  conditions  as  under 

1.  F.  B.  Laney,  loc.  oit.  p. 


---    -•'•  f  fl>  *-• 


»3 


Jber 


. 


386. 


secondary  cannot  be  denied.  Although  chaloocite  fortred  at 
temperatures  above  31°C.  is  isometric,  it  alters  to  the  usual 

orthorhombio  form  when  cooled,  unless  it  contains  over  8$ 

1 
dissolved  oupric  sulphide.   With  this  one  exception,  the 

synthetic  material  formed  at  higher  ten^eraturef  is  identi- 
cal with  natural  chaloooite.  Consequently  it  seems  that  there 
is  little  hope  of  distinguishing  primary  and  secondary  prod- 
ucts by  microscopic  means,  where  both  are  in  ordinary  re- 
placement relations  to  the  bornite,  and  the  lack  of  break 
between  definitely  recognized  secondary  chalcocite  and  the 
ohaloooite  associated  with  the  graphic  structures,  as  at 
Engels,  cannot  be  regarded  as  a  convincing  argument  against 
the  primary  origin  of  the  latter. 

Hypothesis  of  Selective.  Replacement.  In  the  dis- 
cussion of  the  properties  of  graphic  structures  on  preceding 
pages,  the  features  were  emphasized,  which  oppose  the  hypoth- 
esis of  simple  replacement  origin,  for  the  ohaloooite.  In 
resun.e',  the  occurrence  of  chaloooite  in  the  graphic  structure 
often  in  the  heart  of  bornite  grains,  the  lack  of  dependence 
of  the  chalcoolte  on  gangue  boundaries,  the  abrupt  termina- 
tion of  graphic  areas  against  masses  of  bornite,  and  the 
contrast  to  the  forms  of  ordinary  replacement  are  difficult 
to  explain  as  results  of  rnetaaomatio  processes  and  offer 
very  eerioua  objections  to  the  hypothesis. 

1.  Eugen ,  Posnjak,  E.  T.  Allen,  H.  E.  Iterwin,  loo.  ait. 


tft*« 


9*lC 


.•Ub   ';  ^ 


r  erf  *o 


* 


So  e: 


i    « sfe$. 
i 

' 

' 
- 


389. 


On  the  other  hand  it  will  be  recalled  that  there 
is  fairly  definite  evidence  that  the  development  of  ohaloo- 
oite  in  many  graphic  structures  was  actually  accompanied  by 
replacement  reactions.  The  microscopic  evidence  has  al- 
ready been  summarized.  There  are  a  few  cases  in  which  the 

1 
field  evidence  is  significant.  At  Bisbee,  Arizona,   and 

2 
at  Rising  Star  Mine,  Shasta  Co.,  California,   the  ohaloo- 

oite  ores,  in  which  the  graphic  structures  occur,  appear  not 
'to  oontinue  into  the  deepest  workings.  Although  the  deposits 
have  not  been  examined  with  sufficient  thoroughness  to  es- 
tablish these  observations  as  definitely  as  might  be  desired, 
the  evidence  is  very  suggestive  that  the  ohaloooite  in  the 
graphic  structures  is  related  to  the  distribution  of  secon- 
dary enrichment.  At  Butte,  when  the  deepest  known  ores  con- 
taining ohaloooite  and  bornite  in  the  graphic  relation 
occur,  definite  observations,  which  will  not  be  detailed 
here,  make  it  apparent  that  from  our  present  information, 
the  possibility  of  the  secondary  alteration  of  bornite  even 
at  the  great  depths  attained  there  cannot  be  eliminated. 

If,  however,  the  replacement  hypothesis  is  to  be 
ade  acceptable,  it  must  be  modified  in  such  a  way  that  it 
offers  a  competent  explanation  for  the  peculiar  but  particu- 
larly characteristic  features  of  graphic  structures,  which 
are  so  suggestive  of  contemporaneous  nature  of  the  mineral 


1.  See  pages  173-17^. 

3.  L.  C.  Graton,  personal  communication. 


rff     .8 
.fct 


erf* 


9qac 


390, 


components.  An  explanation  must  be  offered  for  the  selective 
manner  in  which  the  bornite  is  attacked,  in  which  the  obvious 
lines  of  weakness  are  neglected  aud  the  advance  of  the  ohal- 
oocite  is  directed  into  irregular  yet  fairly  constant  forma. 
To  do  this,  the  hypothesis  nay  be  suggested  that  bornite  ex- 
ists in  two  forma,  one  of  which  is  slightly  more  resistant 
to  the  influence  of  enriching  prooeaaea  than  the  other.  The 
difference  between  them  is  assumed  to  be  sufficient  to  allow 
one  form  to  remain  unaltered  under  certain  conditions  of 
mild,  probably  long-continued  attack  of  altering  solutions, 
while  the  other  is  completely  replaced.  According  to  this 
hypothesis,  tne  graphic  atruoture  would  be  explained  as 
the  result  of  selective  replacement  of  the  more  easily 
altered  type  of  bornite  from  a  contemporaneous  intergrowth 
of  the  two.  The  replacement  relations  usually  associated 
with  the  structure  could  be  readily  considered  to  be  slight 
modifications  of  the  rr.ore  resistant  type,  or  leas  commonly 
incomplete  replacement  of  the  feebler  type.  As  replacement 
tendencies  beo&me  more  and  more  intense,  the  resistant  type 
would  be  expected  to  yield,  ai>d  the  earlier  structure  would 
be  gradually  obliterated. 

This  explanation  harmonizes  the  conflicting  evi- 
dence in  a  very  complete  way.  The  evidence  o£  contemporane- 
ous intergrowth  structures  may  be  accepted  without  modifica- 
tion, for  they  are  attributed  to  the  primary  relations  of 
the  two  bornites,  and  the  evidence  of  replacement  relations 


-At?    &J 


JB  a  fa 


-  . 


'-    •'*; 


IHl  *-• 


»o*j 


391. 


between  the  bornite  and  ohalcocite  oar,  also  be  taken  without 
discount  for  a  aetu.aon.atio  origin  ia  assigned  to  the  ohaloo- 
oite . 

The  conception  of  two  bornitee,  of  course,  cannot 
be  confined  to  the  graphic  structure  alone,  but  must  find 
support  in  features  observed  in  other  forms  of  bornite  re- 
placement.  In  the  diacuasion  of  the  lattice  structure,  the 
oocurenoe  of  plates  or  splines,  apparently  more  resistant 
than  the  surrounding  material,  was  described.   (Figures  lJj-3, 
1 117,  111?,  .and  1^9 . )  ,    In  these  associations  it  is  usual- 
ly evident  that  the  surrounding  material  had  been  altered 
to  a  ohalcooite -bornite  lattioe  with  remarkable  uniformity 
throughout  the  field,  while  the  splines  of  bornite  had 
been  attacked  only  slightly  or  not  at  all.  This  relation 
when  considered  in  the  light  of  the  new  hypothesis,  at 
o:.ce  suggests  that  the  splines  of  bornite  are  of  the  more 
resistant  sort,  while  the  tain  field  was  of  the  easily 
altered  type.  The  relations  in  the  lattioe  structure  would 
indicate  that  widespread  replacement  of  bornite  by  chaloo- 
oite  in  this  pattern  is  irobably  confined  to  the  leas  resist- 
ant bornite.  The  symmetrical  relation  of  the  splines  to 
the  surrounding  lattice  shows  that  the  two  bornites  are 
either  of  the  same  or  closely  related  crystal  forms.  In  a 
few  instances  a  slight  development  of  chalcooite  in  the 
lattice  form  has  been  observed  along  the  rrargins  of  the 
splines,  and  it  is  probable  that  the  same  structure  exists 


. 


. 


393* 


in  them  as  in  the  surrounding  field,  but  that  it  is  less 
easily  developed  by  the  chaloooite. 

Two  forms  of  bornite  associated  in  the  graphic 
pattern  would  suggest  contemporaneity  in  formation,  but 
splines  of  the  more  resistant  type  surrounded  by  the  easily 
altered  sort  would  suggest  an  age  difference  which,  however, 
might  be  only  slight.  Two  interpretations  tray  be  offered 
for  the  second  relation: 

(l)  that  the  splines  were  the  earliest  to  font, 
probably  as  skeletal  crystals,  a;.d  that  the 
angular  spaces  between  then  were  filled  later 
by  the  less  resistant  bornite,  or 

(&)  that  the  easily  altered  type  was  the  first  to 
fora,  and  was  then  altered  to  the  resistant 
bornite  by  agencies  which  advanced  along  crys- 
tallographio  planes,  similar  to  the  invading 
chalcooite  at  a  later  time. 

In  our  present  lack  of  knowledge  concerning  the  possible 
nature  of ' the  differences  between  the  two  bornites,  if 
suoh  exist,  further  speculation  is  useless,  but  it  may  be 
suggested  that  since  the  earliest  product  to  crystallize 
from  a  melt  or  solution  is  often  the  purest,  as  has  been 
suggested' by  Dr.  E.  T.  Allen,  and  since  pure  substances  are 
usually  more  resistant  to  alteration  than  impure,  the  first 
interpretation  is  probably  the  better  one. 

The  sharply  defined  spots  and  irregular  blebs  of 
bornite  which  remain  as  residues  in  ohaloooite  derived  by 
lattice  replacement,  also  indicate  the  existence  of  a  more 
resistant  type  of  bornite  than  that  which  had  surrounded 


.  «4 


393. 


their.   Their  clean  boundaries,  irregular  forma  and  distribu- 
tion pl-oe  then,  la  contrast  to  the  usual  angular  residues 
produced  by  a  lattice  replacement .  Their  resemblance  in  some 
cases  to  forms  of  graphic  structures  suggests  that  the  ohal- 
cocite  of  the  latter  cay  have  been  derived  by  lattice  replace- 
ment . 

Chaloooite  occurring  in  the  graphic  relation  in 
the  heart  of  bornite  grains  has  been  mentioned.  Similar 
cases  in  which  the  outer  portions  of  grains  are  apparently 
more  resistant  than  the  cores  have  been  observed  in  which 

the  bornite  in  the  centre  is  partially  replaced  by  chaloo- 

1 

cite  in  the  lattice  pattern  (Fig.lW  ),  which  may  be  ac- 
cepted as  proof  that  the  chaloooite  is  of  metasomatio  origin. 
The  resistance  of  the  bornite  in  the  outer  parts  of  the 
grains  is  probably  to  be  explained  in  the  same  way  as  the 
survival  of  the  splines,  which  has  just  been  discussed. 
It  is  worthy  of  note  in  this  connection  that  the  centers  of 
bornite  grains  on  the  polished  surface  frequently  are  strong- 
ly attacked  by  artificial  reagents,  whereas  the  marginal 
portions  are  more  resistant. 

The  lattice  structure  as  has  been  mentioned  has 
been  observed  well  developed  throughout  definite  grains  of 
bornite,  yet  sharply  terminated  against  others  which  are 

1.  J.  Murdoch,  Op.  cit,  Frontispiece,  Fig.  2. 
It  should  be  noted  that  the  specimen  illustrated  is  a  crystal 
determined  to  be  of  proper  bornite  form.  This  removes  the 
possibility  that  the  outer  rim  of  bornite  is  of  later  deposi- 
tion. 


10  *>     i 


apparently  untouohecl  by  the  ohalcooite.  The  strips  of  ohal- 
oooite  are  out  off  as  regularly  and  as  consistently  as  if 
against  another  mineral  possessing  complete  resistance  to 
their  attack.  This  selective  replacement  is  usually  more 
apparent  in  the  early  stages  of  enrichment,  but  it  has  been 
seen  in  specimens  in  which  ohaioocite  is  the  predominant 
member  of  the  lattice.  Such  differential  attack  cannot  be 
attributed  to  chance,  and  it  seens  to  imply  that  there  is 
a  variation  in  the  resistance  of  bornite  which  can  direct 
and  control  the  advance  of  chaloooite  alteration  in  the 
early  stages. 

In  ores  from  the  Koteina  District,  which  have  been 
described  previously  (page  241),  the  development  of  the 
secondary  copper  sulphides  was  found  to  leave  certain  ir- 
regular strips  and  bands  of  bornite  untouched.  The  ohalco- 
oite and  oovellite  seem  confined  to  definite  grains,  but 
usually  permeate  the  bornite  thoroughly  within  these  limit*. 
They  rarely  assume  the  lattice  pattern.  In  the  zones  re- 
sistant to  ohalcooite,  however,  ohaloopyrite  ia  developed 
sparingly,  but  in  excellent  lattice  forms.  If  the  theory 
of  two  bornites  is  applied,  the  relation  indicates  that 
both  sorts  are  capable  of  yielding  lattice  replacements,  an 
observation  which  finds  confirmation  elsewhere.  The  forma- 
tion of  ohalcopyrite  may  be  attributed  to  the  accumulation 
of  iron  released  by  the  replacement  reactions  which  pro- 
duced the  ohaloooite  and  oovellite  in  the  more  easily  al- 


rff 


.til 

10     •£ 


jbadi 


395* 


tared  boraite. 

On  polished  surfaces  of  ores  from  the  Bonanza  and 
Jumbo  Mines,   at  Kenneoott,   angular,  broken  strips  of  bornite 
have  b@an  observed  in  chaloopyrite.     When  first  examined, 
they  at  once  give  the  impression  that  the  bornite  is   in  veins 
in  the  ohalcopyrite  and  should  therefore  be  considered  to  be 
of  later  age*     If  a  strip,   however,    is  traced  into  a  bornite 
field,    its  continuation  is  usually  marked  by  a  delicate  bor- 
der of  fine   ohaloopyrite  spines,  which,  gradually  disappear 
and  allow  the   "vain"  to  lose  its  identity  in  the  massive 
/^bornite.      (Fig.    123  , 12fc  ,  \K%  ,   and  1^.)       The   summation 
of  the  evidence    (page     259)     presents  such  a  strong  case 
favoring  the  replacement  of  bornite  by  ohalcopyrite,    that  it 
forces  the  belief  that  the  bornite   strips  are  not  veins  in 
the  chaloopyrite,   but  residues,   which  for  some  reason  remain 
resistant  to  chaloopyrite  attack,   as  do  the   splines  in  the 
lattice   structure   to  the  chaloocite  attack. 

Direct  evidence  indicating  two  types  of  bornite, 
however,    is    not  abundant  or  especially  significant.     There 
is  a  slight  variation  in  the  hardness  of  different  grains 
or  different  parts  of  the  same   grain  in  some  oases,  which  is 
shown  most  definitely  by  the  relief  on  the  polished  surface. 
Variations  of  this  sort  may  possibly  be  attributed  to  changes 
in  properties  due   to  crystallographic     orientation     but  not 
very  probably    for  the  mineral  is     isometric.     Harder  rims 
about  softer  cores  have  been  observed  in  material  from  the 


. 


>        396, 


1 
Kuskulana  District,  Alaska,   but  in  no  cases  have  the  hard 

and  soft  types  been  found  associated  in  forms  similar  to 
graphic  patterns.  An  array  of  arguments  against  the  view 
of  two  types  of  bornite  may  be  raised,  but  none  of  very 

positive  or  final  character.  The  oolor  of  bornite  on  the 

2 
freshly  polished  surface  is  very  constant.    Slightly 

yellowish  tints  have  been  observed,  but  it  is  most  probable 
that  they  are  due  to  submiorosoopio  spines  of  ohaloopyrite, 
and  not  to  any  variation  in  the  character  of  the  bornite. 
The  etch-patterns  of  bornite  offer  no  evidence  of  two 
types  possessing  different  properties.  The  cracks  are  either 
of  the  regular  sort  within  the  limit  of  the  grain,  or  evenly 
irregular  in  a  way  which  carries  no  significance.  Strips 
with  a  slight  variation  in  pattern  have  been  observed  in  a 
very  few  oases,  and  may  possibly  be  related  to  the  splines 
but  the  orientation  of  the  cracks  suggests  that  the  structure 
is  probably  due  to  twinning.   Bornite  front  different  dis- 
tricts often  shows  a  distinct  variation  in  vigor  of  reaction 
with  the  ordinary  reagents,  but  the  character  of  the  re- 
action seems  to  be  constant  within  a  limited  field,  as  far 
as  our  experience  goes.  The  regional  variation  may  be  due 
to  purely  physical  characters,  such  as  porosity,  or  the 
presence  of  submiorosoopic  impurities. 

The  constant  ohercioal  character  of  natural  borr.ite 


1.  A.  Wandk/e,     personal  communication. 

2.  J.   Murdoch,     Op.    Cit.,    p.    35. 


397. 

may  be  regarded  as  established  with  oertuinty  and  there  is 

little  likelihood  that  true  variations  in  composition  ooaur. 

1 
In  work  done  in  the  .Geophysical  Laboratory,   which  will 

aoon  be  published,  bornite  was  found  to  possess  no  inversion 
points,  suoh  as  the  one  between  isometric  and  orthorhombio 
forms  of  ohaloocite  at  91°.  As  far  as  the  properties  of 
bornite  are  known  at  present,  there  is  no  definite  physical 
or  chemical  evidence  to  indicate  that  it  possesses  more  than 
one  form* 

A  rather  serious  objection  which  may  be  raised  a- 
gainst  the  hypotheses  of  selective  replacement  is  that  no 
partial  stages  of  the  metasomatio  changes  have  been  observed 
with  certainty.  Bornite  residues,  in  the  lattice  pattern  in 

some  oases,  should  be  found  in  the  ohalcooite  blebs  and  lobes 

the 
of  the  graphic  structure  according  to  mechanism  suggested, 

but  in  general  they  are  completely  absent.   Indications  of 
residual  bornite  strips  have  been  found  in  two  or  three  cases, 
but  the  evidence  is  hardly  definite  enough  to  settle  the 
matter,  and  although  it  lessens  the  weight  of  the  objection, 
it  does  not  remove  it*  Chalcocite  which  has  been  formed  by 
the  lattice  replacement  of  bornite  usually  possesses  a  tri- 
angular etoh-pattern;  the  ohalcooite  in  the  graphic  structures 
commonly  yields  the  orthorhombio  etch-cleavage.   (Fig. 136.) 
Consequently  it  is  improbable  that  the  ohalcooite  in  the 

1.  Private  communication. 


v  O      V 


398. 

graphic  relation  with  bornite  was  derived  by  a  replacement 
of  the  lattice  type,  which  greatly  weakens  the  force  of  one 
attractive  explanation  acoording  to  the  hypothesis  of  selec- 
tive replacement. 

Summary  and  Conclusion 

The  relations  of  the  ohaloooite  and  the  bornite  in 
all  graphic  structures  cannot  be  satisfactorily  explained 
either  by  the  hypothesis  that  the  two  minerals  are  of  strict- 
ly contemporaneous  origin  or  by  the  view  that  the  ohaloooite 
is  produced  by  simple  replacement  of  bornite  of  ordinary 
homogeneous  character.  Normal  replacement  origin  will  appar- 
ently not  account  for  any  of  the  graphic  structures.  Strict- 
ly contemporaneous  origin  will  account  for  many. 

If  the  hypothesis  of  contemporaneous  origin  is 
modified  to  the  extent  that  it  allows  replacement  conditions 
of  bornite  by  oh&loocite  to  follow  upon  the  heels  of  contem- 
poraneous deposition,  it  becomes  nore  acceptable,  and  affords 
a  satisfactory  explanation  for  nearly  all  the  observed  fea- 
tures. The  ohaloocite  fits  into  the  later  phases  of  the  se- 
quence of  primary  minerals  in  a  logical  manner  and  chemical 
knowledge  of  its  properties  and  range  of  stability  offers 
no  objections  to  its  being  placed  there.  Opposed  to  the 
view  is  the  difficulty  of  finding  any  break  between  second- 
ary ohalcooite  and  that  closely  and  intimately  associated 
with  the  graphic  structure.  In  two  districts,  the  apparent 


T 


MN>JtflA£ 


'      JO     8«£ 

IdDtaftdc  ••.*r.i";,.--; 

-ello 


x«  • 


399. 

dependence  of  chaloocite  in  the  graphic  structure  on  the 
distribution  of  the  secondary  sulphides  is  also  of  opposing 
nature,  but  more  careful  field  and  laboratory  studies  are 
required  before  the  strength  of  the  last  argument  oan  be 
fully  established.  If  it  is  upheld  by  more  detailed  work, 
it  offers  a  serious  objection  to  the  hypothesis  of  primary 
chaloooite. 

The  explanation  baaed  on  selective  replacement 
depends  on  the  strong  evidence  of  the  importance  of  metas- 
ometiam  in  close  association  with  the  graphic  structure,  and 
the  apparently  greater  resistance  of  boraita  in  certain  forms 
to  alteration.   It  finds  support  in  the  satisfying  way  that 
it  harmonizes  conflicting  evidence  of  contemporaneous  and 
replacement  structures,  and  in  the  many  features  in  other 
structures  which  suggest  two  types  of  bornite,  such  as 
splines,  rime,  resistant  specks  and  abrupt  terainations.   It 
is  seriously  weakened  by  the  lack  of  known  chemical  or 
pronounced  physical  variations  in  the  properties  of  bornite, 
ani  by  the  rareness  of  any  forms  which  may  be  interpreted  as 
latex-Mediate  stages  in  the  replacement. 

Of  the  four  possibilities  which  have  been  suggested, 
viz.  striotly  contemporaneous  deposition,  modified  contempo- 
raneous depostiion,  selective  replacement  and  normal  replace- 
ment, it  is  noteworthy  that  the  first  three  agree  in  assum- 
ing that  the  graphic  pattern  Is  controlled  by  a  primary 
structure  in  the  ore,  and  each  of  these  iieas  has  something 


ao  fl,eo 

,^ao« 


i«*       {*•'<?. 


400. 

to  commend  it,  whereas  the  fourth  hypothesis  which  assumes 
no  prior  control,  finds  no  positive  support  from  any  of  our 
observations* 

The  importance  of  the  solution  of  the  problem  is 
not  to  be  minimized,  for,  if  either  hypothesis  of  contem- 
poraneous origin  can  be  accepted,  it  establishes  the  ex- 
istanoe  of  primary  ohaloooite;  if  they  are  discarded,  the 
chief  argument  favoring  the  primary  nature  of  the  ohaloocite 
in  many  deposits  (such  as  Engles  for  example)  is  lost,  and 
the  assumption  of  two  ages  of  ohaloocite  becomes  unwarranted* 

It  is  apparent  from  the  evidence  and  arguments  ad- 
vanced that  none  of  the  hypotheses  can  yet  be  accepted  with- 
out reservation  nor  can  either  one  of  them  except  that  of 

normal  replacement  be  eliminated  from  the  field*  Neither  is 
it  certain  that  any  one  affords  the  sole  explanation  of  the 
graphic  structure.  If  the  relative  values  of  the  different 
arguments  could  be  established,  the  question  wouli  be  easy 
to  settle,  but  in  the  present  state  of  our  knowledge,  this 
is  difficult  to  do.  With  the  constantly  increasing  tzass  of 
information  which  is  being  made  available  from  geological 
and  chemical  work,  however,  it  is  quite  probable  that  before 
long  a  closer  appraisal  of  the  evidence  may  be  made  and  a 
final  answer  given,  but  at  present,  the  wiser  course  is  to 
leave  the  problem  open. 

The  question  of  the  origin  of  the  graphic  structure, 
although  perhaps  the  most  fascinating  and  puzzling  encountered, 


':.  .UCfC 


OBO 


to   w 


•i  .i. 

s^netm^xk 
-d^.i«v  fcafq; 

>oa,&  a 

ei  le 
•di  : 


401, 


is  merely  one  of  the  many  problems  which  seek  to  oonoeal 
themselves  behind  the  intricate  structures  and  varied 
behavior  of  bornite,  toward  the  solution  of  which  this 
contribution  has  been  directed* 


MICE  OPHQgQGHAPHS 


BOOKS 


402. 


PLATF  TIV 

Photographs  of  Polished  Surfaces  of  Ores  from  the 
Evergreen  fr'ine,  Colorado.' 


Fig.  kl.   (x  fO  diameters)  Bornite  (gray),  partially 
replaced  by  chalcocite  (light),  working  in 
from  gangue  (black)  contacts.  The  feathery 
material  in  fine  lattice  orientations  in  the 
chalcooite  consists  of  very  fine-grained 
ohalcopyrite,  ani  a  little  bornite,  with 
some  oxidized  material.  For  discussion  see 
page  75.  A  grain  of  ohalcopyrite  (light) 
in  the  lower  part  of  the  field  remains  re- 
sistant to  the  ohfllcocite. 


Fig.  &2.   (x  £0  diameters)  Chaloopyrite  (light)  and 
bornite  (dark).  In  part  associate.!  with 
mutual  boundaries  and  in  part  in  relations 
indicating  replacement  of  chelcopyrite  by 
bornite.  Some  spines  of  secondary  ohalco- 
pyrite penetrating  the  bornite  from  gangue 
(black)  contacts. 


Fig.  k-3.   (x  £0  diameters)  Primary  chalcopyrite  (light; 
opj)  an-!  bornite  (dark)  with  mutual  boundaries. 
Bornite  partially  replaced  by  secondary  chaioc- 
pyrite,  in  imperfect  lattice  structures,  sc- 
oompanied  by  shrink 9 ge  cracks. 


Fig.  Wt..   (x  ^0  diameters)  Bern!  te  (gray)  with  a  little 
primary  chalcopyrite  (light;  cpj)  in  smooth 
grains  and  more  abundant  secondary  chplcopyrite 
(op?)  developing  at  the  expense  of  the  bornite 
as  rirr:3  along  gangue  veinlets,  or  as  imperfect 
lattices. 


45-- 


40  3  v 


PLATS  XV 
Photograph  a  of  Polished  Surfaces. 


Fi<".  ^5.      (x  "0  diameters)     l^vergreen  Mine,   Colorado.     Bor- 
nite    (lark)  and  chalcopyrite   (lisht)   in  graphic 
association. .    Secondary  chalcopyrite  developed 
in  bornlte  bleb  in  up  jar  portion  of  the   fiel^. 
Note  shrinkage  crooks . 


FJP.   '-6.      (x  50  diameter 3)     ^^rgreen  Mine,  Golorn,''o.     Bor- 
nlte   (dark)  be  "h  in  graphic   structures  -.vith 
chalcopyrite  alonp;  ganifue  veinlets   (blp.ck). 


Fig.  k-7.      (x  meter  a)     Evergreen  Mine,   Colorado.     Bor- 

nite   (dark)  both  in  mutual  relations  vrtth  chalco- 
pyrite    (lir^ht)   -n1  as  a   replocement  of  chalco- 
pyrite ir.  definite  veinleta. 


Fig.  Itf?.      (x  ?0  diametera)     Evergreen  Mine,   Colorado.     Bor- 
nite    (dark)   in  mutufl  relations   toward  primary 
chalcopyrite   (light)   --ni   t  lly  replaced  by 

secondary  chplcopyrlte,   in  the   lattice  structure. 


Fig.  k9.      (x  !!0  diameters)     Marble  Bay  Mine,  Texade   lalani, 
B.   C.     Bornite   (gr->y)  with  an  intemiittant  border 
of  klaprotholite    (?)    (white),   and   ohr.lcoci te, 
and  chalcocite   (li^ht  gray).     Qpngue ,  t 


Fi?.   50.      (x  kg  diameters)     Merble  Bay  Mine,   Texnda   Inland, 
B.   C.      Chalcopyrite    (li^ht)   anl  bornite    (dark) 
in  primary  relations  which   nli^htly  surest  the 
earlier  sp,9  of  the  chnlcopyrite. 


404, 


PL/TS  7 VI 

PhQtosraph-3   of  Thin  ^eoti one   of  Hooka   and  Ores 
froii;   ^n.^;-:;,   Ce.ll  fornjp. .. 


Fig.  51.  (x  20  diameters)  Bornite  (bL?ok)  raplaoing 
rock-roinarala,  with  only  slight  development 
of  chlorite  end  epldote. 


Fig.   5?.      (x  90  diameters)     Creased  niools.     Broken 
apatite  prism  in  strained  feldspar. 


Fig.   53.      (x  n.  diameters)     Creased  niools.     Feldspar 
phenooryst  in  granodiorite  porphyry,   showing 
recrystallization. 


Fig.   5^.      (x  20  diameters)     Same   aa  in  figure  "?,  but 
without  crossed   niools. 


Fig.   55.      (x  16  diameters)     PoikJlitJc  hornblende,  with 
feldspar   qni  magnetite  chiefly. 


f^,   56.      (x  "*>?  diameters)     ^uh^dral  epidote  crystals 
included  in  bornite. 


0 


I 


•%•>,.... 


•.- 


405. 


PUTS  XVII 


Photographs  of  Thin   Seotiona  of  Hooka   ond  Ores 

from  ffngels  t  Call  for  ni  P  .' 


Fig.   57. 


(*  67  diameters) 

by  magnetite  ani 
Chlorite  also  in 


Hornblende 
corroded  by 


oryatal  surrounded 
chlorite  end  bornite, 


thin  foils  between 


magnetl te 


. 


(x  ~?  diameters)     M?p;netite  r?n^  bornite  with 
chlorite  along  boundaries  of  magnetite  grnina. 


(x  "°  dimeters)     Hornblende  cryetsla  containing 
amell  inclu^ion^  of  magnetite  end   spin   1   (spn.), 

\  corroded  by  surrounding  magnetite.     Note 
abundant  inclusions  of  apatite  in  the 
and   the  occurrence  cf  bornite  in  ragged 
with  chlorite. 


Fig.  60.     (x  6"  diameters)     Bornite  en1  chlorite  replacing 
aaphibole.     Magnetite  in  smoother   grains. 


*  ** 


406. 


PLATE  XVIII 
Photographs  of  Ores  from  Engels.  California. 


Fig.  6l.   (x  30  diameters)  Thin  section.  Hornblende  par- 
tially replaced  by  calcite,  developed  along 
cleavages  of  the  former. 


Fig.  6?.   (x  5^  diameters)  Thin  section.  Crossed  nicols. 
Plazioolase  partially  replaced  by  heulandite 
(heii.).  and  both  partially  altered  to  natrolite 
(white). 


Fig.  63.   (x  Ui  diameters)  Thin  section.  F.piiote  vein- 
lets  cutting  albite  and  chlorite.  Also  patches 
of  epidote. 


Fig.  6^.   (x  *iO  diameters)  Polished  section.  Bornite 

(white)  with  inclusions  of  chlorite  (dark 

laths);  magnetite  (gray),  earlier  than  the 
borni  te. 


Fig.  6~.   (x  65  diameters)  Thin  section.  Ore-bearing  di 
orite  cut  by  siderite  (?)  veinlets. 


Fig.  66.   (x  650  diameters)  Polished  section.  Chlorite 
(dark)  corroded  by  borni te. 


407 


PLAT*  TIX 

Photogrpphg  of  Polished  Surfaces  of  Ores 
""from  ffngels,   Cali  " 


Fig.   67.      (x  7^  Hamsters)     Chalcopyrite   (dark)  and 
bornite   (light)  with  mutual  boundaries. 
(Jangue,  black. 


Fig.   6g.      (x  50  diameters)     Bornite   (gray)  with  chalco- 
oite   (light)  developed  along  chlorite   (dark) 
inclusions   in  the  bornite,   *n^   slong  gangue 
veinlets. 


Fig.  69.      (x  60  diameters)     Bornite   (dark)  with   chaloo- 
oite   (li£ht)  both  in  graphic  associations 
(upper  part  of  ths  field^   and  in  veinleta,  in 
part  alon?  gangue   (black)  contacts. 


Fig.   70.      (x  70  diameters)     Bornite   (d?rk)  with  chalco- 
olte   (lir,ht)  moatly  in  graphic  structures. 
Note  the  extension  of  bornite  through  the 
field  alonp,  the  intem-ittant   line  of  gangue 
(blaok),  with  ohalcooite  along  the  centre  of 
the  bornite  sone. 


Fig.   71.      (x  '"^  diametera)     A  oortion  of  the  field  of 

Fitr.   70. 


Fig.   72.      (x  650  diameters)     A  portion  of  the  field  of 
Fig.   70.     Bornite,  dark;   chplcocite,    light. 


•• 


408. 


PLATE  T* 

Photographs  of  Poliahad  Surfaces  of  Ores 
"ffngela,   Ca     fornia." 


Fig.   7^>.      (x  SO  diameters)     Veinlets  and  patches  of  chaloo- 
cite    (light)   in  bornite    (dark)    in  part  along 
gangue  veinlets   (black).     Note  subgraphio  form 
of  some  of  the  larger   chalcocite  grains. 


Fig.  71*.      (x  70  diameters)     Bornite  grains   (lark)  with 
riirs  of  chalcocite   (light)  in  altered  diorite 
(darker  gray).     Note  the  imperfect  beginnings 
of  B   lattice  structure  between  the  bornite 
and  chalcooite  in  many  places. 


Fig.   71.      (x  70  diameters)     Bornite   (dark)  with  chalco- 
oite  (light)  both  in  graphic  structures  and 
in  veinlets  in  the  bornite.     Note  cleavage 
cracks  in  the  ohalcocite  about  holes  in  the 
polished  surface. 


Fi?;.   76.      (x  2?0  diameters)     Field  of  ch^lcocite   (white) 
cut  by  a  gangue  veinl«t   (carbonate;  black), 
with  bornite   in  intermittant  strips  alonp:  the 
margin  in  the  chaloooite. 


409. 


PLAT?!  7X1 

Photographs  of  Polished  Surfaces  of  Ores 
rrom  Angels,  California . 

(Photographs  on  this  page  by  W.  L.  Whitehead.) 


Fig.  77.  (x  "0  diameters  1)  Bornite  (dark)  cut  by 
lattice  of  chalcopyrlte  (li^ht).  Gangue, 
black. 


Fig.  7  .   (x  900  diameters)  Lattice  structure  developed 
in  bornite  (bn)  by  chalcopyrite  apines  (op) 
with  chalcocite  rims  (cc). 


Fig,  79.   (x  300  diameters  ?)  Bornite  (dark)  and  cha loo- 
cite  (light)  in  graphic  structure. 


Fig.  £0.   (x  1000  diameters)  Photograph  at  higher  magni- 
fication of  portion  of  the  same  bornite-chalco- 
oite  graphic  structure  as  shown  in  Fig.  79* 
Note  that  the  chalcooite  is  dark  and  the  bornite 
is  li?ht,  due  to  a  different  color  screen  used 
in  this  case.  The  similarity  in  appearance  of 
the  two  photographs  in  spite  of  this  change  is 
a  striking  example  of  the  mutual  relations  of 
the  two  minerals. 


410, 


PLATS 

Photographs  of  Polished  Surface 3  of"  Ores 

from'  "FngeTg,  California,. 


Fig,  £l.  (x  250  diamatara)  Bornite  (dark)  and  chaloo 
oita  (light)  in  a  fine-grained  graphic  atruo 
tura.  Apportion  of  the  field  shown  in  Fig. 


Fig.  £2.      (x  250  diameters)     Chalcooite   (light)  in 

graphic  structure,  rear  edge  of  bornite  grain 
(dark),   end  also  rasooiated  vr j  th  bornite  in 
vainleta  and  along  gan^ue  oontact. 


Fig.  f!T.      (x  2^0  di&metero)     Intsrgrowth  of  magnetite 
(li»ht)  ?n1   ilroenite,   in  whicjh   the   letter  is 
largely  altered  to   leucoxer.3   (gray).     A 
little  hematite   (white). 


Fig.  ?k.      (x  70  -!laweter8)     Superior  Mine,  Angels,   Cali- 
fornia.    Magnetite  grains   (shadowed,   due  to 
M?h  relief)  set  in  a   field  of  bornite    (dark) 
with  chalcocite   (light)   in  graphic  associations 


411 


PLATE  XXI I I 

Phptogr?  pha  •  of  Pol  i  3hnd  3ur  f  r-  peg  of  Or  eft 
frog   the   3ur:^r  JOT  MJn? ,   near   Tnsrels,    California 


Fig.   £5.      (x  70  diameters)     Magnetite  crystals,   shown  by 
black  bordara  due  tc  hip;h  relief,   surrounded 
by  bornite   (dark)  containing  blebs  of  chaloo- 
oi te    (light)   suggesting  imperfect  graphic 
structures    (subgraph! c  otrrotures).  t 


Fig.   ?6.      (x  70  diametera)     Magnetite  grains,   shown  by 

black  borders,  with  small  inclusions  of  bornite, 
and  surrounded  by  bornite  containing  chalcccite 
in  graphic  form.     Inclusion;-?  of  ohlorlte  in  the 
bornite.     Note  lr ok  c  >ndence  of  the  chaloo- 

oite  on  marp;in<  of  the  bornite  areas  or  on  chlo- 
rite inolusiono. 


Fig.  #7.      (x  70  diameters)      3irailrr   to  Fi^. 


Fig.  ££,      (x  70  diameters)     Bornite    (d'-rk)  with  chalco« 
cite   light   in  sra.'hic  structure.     Magnetite 
grains,  shown  by  hifth  relief. 


412. 


PLATE  XXIV 

Photographs  of  Poll ehed  Surf ace  of  Orea 
from   the  Marine.  Mine,   Super  for  7  Arfzona. 


Fig.  £9.  (x  ?50  diameters)  Eornlte  (dark)  in  graphic 
structure  with  galena  (white).  Tetrahedrite 
(td),  light  grey. 


Fig.  90.      (x  250  diameters)     Similar   to  Fi?.   £9.     Note 
great  range  in  oize  of  galena  grains. 


Fla;.  91,      (x  h~  3iametera)     Ohalcopyrite   (white)   p.nd 
bornlta   (a  very   li^ht  p;r*y)  broken  by  later 
minerals   (quarte  in  p-rt). 


Fisr.    .    .      (x   '-~  .Uametero)     Bornite   (dark)   snl  ohsloo- 
pyrf  te    (light)  in  vein-like   strings  of  elon- 
gate i  bleba,  often  around  bornite  grains. 
(The  VRriatlone  in  shade  of  the  bornite  are 
due  to  ataina  on  the  plate). 


93  •      («  Ji-5  diameter  a)     Pyrite   (rough)  broken  by 
veinlets  of  bornite   (smooth). 


Fig.  9ii.      (x  fc"  diameters)     Pyrite    (rou«»h)  broken  by 
veinlets  of  bornite   (darker  sroooth). 

A  primary  replacement. 


413 


PLATE  XXV 

Photoe.ra.oha  cf  Polished  3ur-faoes  of  Ores 
"line.  Superior,  Arizona. 


Fig.  95.   (x  lJ-5  diameters)  Pyrite  grains  broken  by 
bornite  veinlets. 


Fig.  96.   (x  ^5  diameters)  Pyrite  trains  broken  by 
bornite  veinlets.  Replacement  of  pyrite 
more  advanoed  than  in  figures    and  jt. 


Fi".  97.  (x  "?50  diameters)  Chaloooite  etohed  with 
nitrio  pci-l,  revealing  grained  structure 
and  orthorhombio  nature,  ahown  by  the  one 
set  of  strong  craoks  in  each  grain. 


Fig.  9^.   (x  £0  diameters)  Chaloocite  (light)  with 
covellite  (dark). 


/  y      v  ' 

-.  i     -'  v 


414. 


PUTS  XXVI 

:'.          _  __!__  '        __  __.  J2£ 


____         __    . 

fro?     the    KuaVu).  ::•.;-••  -I  :'•?•:  -^  in  f-    "^  : 
""Tfcnrieoott  'Districts,  _  Alaska 


Fig.  99.      (x   50  diameters)     From  flugget  Creek,   Kuskulena 
District.     Bornite    (dark)  and  chaloopyrite 
(light)   in  mutual  relations,  but  with   a  suggestion 
that   the  bornite  is  somewhat   Ipter   than  the  chalco- 
pyrite. 


Fig.    100.      (x  30  diameters)     From  Elliot   Creek,  Kotsina 
District.     Chalcopyrite   (light)   conodedby 
bornite   (dark). 


Fig.   101.      (x  50  diameters)     Nugget  Creek.     Bornite,    (gray) 
'   ch.^lcocite   (white;,  in  aubgraphic  relations. 
be   lack  of  dependence  of  chr>lcocite  distribu- 
tion upon  calcite   (dark  gray)   contacts. 


Fie.   10?.      (x  50  diameters)     Rugget   Creek.     Bornite   (dark) 
with  chslcocite   (light)   in  structures  similar 
to   those  often  shovm  bdtween  chalcopyrite  and 
similar   to  those  often  ohorn  between  chaloc- 
pyrite  and  bornite;  associated  in  on?  small  erea 
in  the  graphic  structure. 


Fi?.   10^.      (x  ^0  diameters)     Bonanza  Mine.     Kennecott. 

Chalcoclte    (light)   elterin?,  to  malachite   (dark) 
along  structural  planes.     /   little  oovellite 
(intermediat?   f^rsy)  between  malachite  and  chalco« 
cite. 

Fig.   10'-.      (x   50  diameters)     Chalcoclte  from       vein  in  th* 
greenstone  near  the  Bonanza  Mine,   Kennecott. 
Partially  altered   to  malachite,  along  parallel 
lines  rsv93lln£  the  orthorhombic  structure.     Com- 
re  with    figure  103  in  whioh   the  chalcocite  de- 

ae  triangular  pattern,  beliaved   to  be 
isometric. 


415. 


PLAT1!!  XXVII 

Photographs  of  Polished  Surfaces  of  Ores 
from  Kennecott,  Alaska* 


105.      (x  £0  diameters)  Bonanza  Mine,     Steely  ohalco- 
cite,  etched  with  nitric  acii.     There  is  a 
alight  suggestion  of  the   concentric   structure 
of   "pebbly  ohaloocite"   in  it,  but   the  material 
la  massive  in  the  handspeciman. 


Fl*.    106.      (x  ?0    Uameters)  Chalcocite,  etched  with  pot- 
assium oy«ni1e   oolution,   showing  variation  in 
grain,   and  the  triangular  etch-pattern  In 
nearly  all  oases. 


Fig.    107.      (x  "0  diameters)     Chalcooite,   etched  with  pot- 
assiufr   cyanide,  revealinrr  triangular  etch- 
pattern. 


Fig.    lOf.      (x  ^0  dlanseters)     Chalcocite,  etched  with  potasaiuc 
cyanide  solution. 

For  discussion  of  these  etch-patter  re,   see  pages    25 
and     356. 


;... 


"7- 


416. 


PLATE  XXVIII. 

Photographs  of  Polished  Surfaces  of  Ores 
from  Ke nne  oo  1 1 ,  A laska . 


Fig.  109.   (x  ?£0  diameters)  Bonanza  Mine.   "Diabasio" 
coTellite.  Covellite  (dark,  mottled  due  to 
pleocroism),  with  chaloopyrlte  (white)  and 
bornite  (bn,  light  gray)  between  the  covellite 

lathe. 


Fig.  110.   (x  2gQ  diameters)  Bonanza  Mine.   "Diabasic" 
covellite.  Covellite  (dark),  with  ohaloo- 
pyrite  (white)  and  bornite  (light  gray)  be- 
tween the  laths. 


Fig.  111.   (x  40  diameters)  Bonanza  Mine.  Covellite 
plates  in  radial  and  scalloped  patterns, 
partially  replaced  by  ohalooolte  (white). 


Fig.  112.   (x  40  diameter)  Bonanza  Mine.  From  the  same 
as  Fig.  111. 


Fig.  113.   (x  65  diameters)  Bonanza  Mine.  Covellite 
(dark)  in  chalcocite  (light). 


Fig.  114.   (x  50  diameters;  Jumbo  Mine.   Complex  ag- 
gregate of  chalcopyrite  ani  bornite  in  con 
centric  structures.   Partially  replaced  by 
-ovellite  an  i  luzonite  (lightest),   Coarser 
plates  of  covelaite  filling  space  cetween 
the  curved  masses  of  ohu-lcopyrite  anl  bor- 
nite. 


417. 


' 


PLAT?  XXIX 

Photographs   of  ?o  11  ?/r  ;H. ; : '    .  :'  Ore  s 

'  from   Ker.i-eoott ,  .Alaak"  . 


Fig.    11^.      (x  2#0  diameters)     Bon^n^a  Mina.     Bornite,  etched 
\*ith  potaaaiur   cyanide  aolution,   yielding  brick- 
like  pattern.     Black  squares  and  rectangles  due 
to  small  blocks  springing  out  between  oraoke  of 
the  etch   structure. 


Fig.    116.      (x  50  dj -:rr.etflra)     'FYie  Minft.     Bornite   (bn)   and 
chalcocite    (cc,   almost   the   aan-iS  color  in  this 
photograph),  ^rtiplly  altered   to  malachite 
vrith  intermediate  covellite  in  the  ohalcocite. 
Ths  triangular  structure  in  the  chalcocite  and 
its  absence   in  the  bornite,    >e  ^hown  by  the 
malachite  veinleta,   ia  notevyorthy. 


Fig.    117.      (x  ?0  liametera)     Bonanza  Mine.     Chplcocite 

altering  to  malachite  alon^   structural  lines. 


Fig.    1121.      (x  £0  diameters)     Erie  Mine.     Bornite   (gray) 
partially  replace'  by  chslcocite    (lip/nt). 
Triangular  structure  shown  by  ;hite  vein- 

leta  in  the  chaloooite,  but  not  in  the  bornite. 
Lack  of  chaloooite  lattice  in  the  bornite  is 
noteworthy. 


Fig.   119.      (x  £50  diameters)     Erie  Mine.     Bornite    (dark) 
partially  altered  to  chaloocite   (light)  along 
irregular  veinletr?.     The  strip^  of  white  chalco- 
cite in  the  blue  type  reveal  the   structure  of 
the  chalcocite.     This   structure  is  emphasized 
by  >  ^laohite  veinleta   (black). 


Fig,    120.      (x  50  diameters)     Erie  Mine.     Bornite   (tn) 

chplcocite    (oc,   somewhat   lighter)  in  a   large 
area  -3   f?»«  veinlete  in  bornite.     Note 

contrast  in  structure  developed  by   the  malachite 
veinlets  in  the  chalcocite  and  in  ths  bornite. 


418. 


PLATE     XXX 

Photographs  of  Polished  Surfaces  of  Ores 
from   Kennecott,   Alaska^ 


Fig.    121.      (x  ^6  diameters)  Bonanza  Mine.     Enargite  oryatala 
in  limestone. 


Fig.   122.      (x  to  diameters)     Jumbo  Mine.     Ch?lcopyrite   (light 
of  the  earliest  generation  partially  replaced 
bornite   (darker). 


Fi«j.   1T5.      (x  2fO  diameters)     -Jumbo  Miiw. 
later  age  developing  in  bornite 
in  feathery  lattice  structure, 


Chalcopyrite  of 
(dark)  in  part 


Fid, 


(x  2?!0  diameters)     Jumbo  Mine.     Chalcopyrite 
(light)  developing  in  feathery  lattice  struc- 
tures in  bornite   (dark).     Note  the  apparent 
reaiatar.ee  of  the  bornite  in  certain  zonsa, 
often  in  vein-like  strips  with  characteristic 
angular  offsets. 


Fig.  125.   (x     •abaters)  Jumbo  Mine.  Bornite  (dark)  re- 
placing chalcopyrite  of  the  earlier  generation 
(light). 


419. 


PLATE  XXXI 

Photographs  of  Polished  Surfaces  of  Orea 
f r  o re  Ke  rine  oo  1 1 ,  At  a  ska. 


Fig.   126.      (x  ?0  diameters)     Jumbo  Mine.     Chalcopyrite 
(light)  of  the  earlier  generation,  partially 
replaced  by  bornite   (dark).     Black  lines  are 
cracks  in  the  polished  surface. 


Fig.   127.      (x  fO  diameters)     Jumbo  Mine.     Similar   to 
Fig.   126. 


Fig.    12g. 


x  fO  diameters)     Bonanza  Mine.     Cove  Hi te 
dark)  partially  replaced  by  ohalcoolte, 
light).     Two  bands  of  bornite  residues 
slightly  lighter)   in  the  ohaloocite.     The 
specimen  possesses  a  scalloped  structure  but 
it  is  not  well  shown  in  this  part  of  the 
field.     Veinlets  of  malachite  in  the  ohaloo- 
oite  in  one  zone. 


Fig.   1?9.      (x  ?0  diameters)     Bonanza  Mine.     Covellite 
(dark)  in  radial  plates,  partially  replaced 
by  ohalooolte . 


-  ^-  - 

-'- 


420. 


PLATE  XXXII. 
Photographs  of  Polished  Surfaces  of  the  Kenneoott,  Alaska. 


Fig.    130.      (x  ?0  diameters)     Bonanza  Mine,  open  out.     Bornite 
(dark)   and  chalcopyrite    (light)   in  scalloped 
structures.     Broken  by  azurite  veinlets. 


Fig.    131.      (x  20  diameters)     Jumbo  Mine.     Complex  associa- 
tion of  chalcopyrite    (light),  bornite    (dark), 
and  covellite    (dark,  mottled)   in  concentric 
structures.     The  covellite  is   largely  a  replace- 
ment of  bornite. 


Fig,    132.      (x  50  diameters)     Bonanza  Mine,  open  cut. 

Chalcopyrite    (light)   and  bornite    (dark)  with 
chaloocite   (a  little   lighter  than  the  bornite) 
in  scalloped  structures. 


Fig.    133.      (x  50  diameters)     Jumbo  Mine. 

Complex  association  of  chalcopyrite  and  bornite 
in  exceedingly  intricate  mass    (mottled,   light 
gray),  with   later  luzonite    (white)   and  covellite 
(mottled  gray),  in  concentric  and   scalloped 
structures  characteristic  of  certain  "knots"   in 
the  ore.     For  discussion  see  page*  zw     to    2<>8  . 
Crystals  of  bornite  or  chaloopyrite  with  rims 
of  luzonite    (?)  occur  on  the  right  side  of  field. 
Covellite  replaces  bornite,  and  forms  fringes 
of  radial  plates  about  margins  of  curved  masses 
of  the  earlier  minerals.     The  luzonite  is  in  part 
later  than  the   covellite. 


421. 


PLAT?  XXXIII 
Photographs  of  Polished  3ur  faces  of  Ores . 

Jr.    lattice 

Fig.    13^.      (x  270  diameters)     Ajo,   Arizona.     Chaloopyrite  A 
structure  in  bornite   (light  gray).     Chaloo- 
oite  and  malachite   (dark  gray)   around   the  mar- 
glna  of   the  grain.     Gangue,  black.     Photographed 
by  Murdoch. 


Fig.   135.      (x  £0  diameters   (?)    )     Virgllina,  Virginia. 
Bornite   (dprk)   with  ohaloocite   (light)  in 
graph! o  structures  and  asoociated  with  mutu- 
al boundaries.     Photographed  by  Murdoch. 


Fig.   136.      (x  50  diameters    (?)    )     Messina,   South  Africa. 
Bornite   (light)   and  ch?>lcocite  (dark)   in 
graphic  structure.     Etched  -#ith  nitric   acid 
showing  the  orthorhoabic  structure  of  the 
ohalcocite,   an-!  its  parallel  orientation 
throughout  the  graphic  pattern.     Photographed 
by  Alfred  Wandtke. 


Fig.    137.      (x  750  liametera)     Ajo,   Arizona.     Bornite   (d.irk) 
with  ohalcopyrite    (light),   devslop»-fl   in  lattic* 
structure.       Photographed  by  l.-lur^och. 


422, 


PUTS  XXXIV 
Photographs  of.  Poll  shad  Surfaces. 


Fig.    13£.      (x  67  diameters)     Messina,   South  Africa, 

Chalcocite  treated  with  nitric  acid  showing 
orthorhorablc  etch-pattern.     Photographed 
by  Alfred 


Fig.    139.      (*  6~  diameters )     Synthetic  chalcocite   from 
the  Geophysical  laboratory  formed  above  91°C 
with  an  excess  of  sulphur,     Etched  with  ni- 
tric acid,  reveallnrr  the  octahedral  parting 
of  the   isometric  hich  temperature  chalcocite. 
This  and   the  following  miorophotographs  are 
by  Murdoch, 


Fig,   UK),      (x  65  diameters)     Synthetic  chaloocite  aimil-r 
to  that  in  figure  139. 


Fig.    1M.      (x  50  diameters)     Similar  to  figures   139  and 


Fig,   1*|2,      (x  70  diameters)     Butte.     Bornite   (dark)    vith 

chalcocite   (lip;ht),  possessing  a  blue  and  white 
lattice,   suggesting  replacement  of  bornite 
through  the   lattice  stage. 


Fig.   1J-3.      (x  ^"  diameters)     Butte.          lines  of  bornite 
(dark)  in  chaloocite   (light)  and  broken  by 
irregular  veinleta  of  ch?,lcooite. 


PU 


433. 


PLATT?  XXXV 

Photographs  of  Poliahad   Surfaces  of   Ores. 

("urdoch) I 


Fig.  l^Jj-,   (x  63  diameters)  Butte,  Montana,  Chaloooite 
(white)  '*ith  bornite  (dark)  along  margins  of 
grains,  and  "spot  structures"  in  the  adjoining 
ohalcocite  grains. 


Fig.  1^5.   (x  3^0  diameters)  Butte,  Montana.  Spot  struc- 
tures, and  subgraph! o  structures,  between  bor- 
nite (dark)  and  chalcocite  (light).  Note 
curved,  banded  distribution  of  the  bornite  specks 


Fig.  lJj-6.   (x  65  diameters)  Butte,  Montana.  Bornite  (dark) 
with  chalcocite  (light)  in  the  lattice  pattern  • 
Note  the  resistant  rim  of  bornite  along  the  left 
of  the  grain. 


Fig,  lfc-7.   (x  65  diameters)  Splines  of  bornite  (dark)  re- 
maining in  a  field  in  which  the  bornite  is 
largely  replaced  by  chalcocite  (light)  in  the 
lattice  structure.  The  splines,  however,  are 
slightly  replaced  by  ch-plcooite  in  veinleta, 
developed  parallel  to  the  lattice  directions. 


Pig.  148.  (at    diameters)  Bonanza  Line,  Zennecott,  Alaska. 
Bornite  (dark)  partially  replaced  by  ahalcopyrite 
flight),  developed  in  the  lattice  pattern.  Bor- 
nite Bplinee  remain  little  altered. 


14-9.  (x    diameters)  Butte.  liontana.  Bornite  spline 
in  a  field  of  bornite-oh:  loocite  luttioe,  show- 
ing incipient  alteration  to  Chaloooite  parallel 
to  the  lattice  directions. 


424 


iOOl 


PUTE  XXX7I 

Photographs  cf  Polisher!  Surfaces  of  Ores. 

(Murdoch). 


Fig.  150.   (x  61  diameters)  Redruth,  Cornwall.  Bornite 
(dark)  along  cracks  in  chalcocite  (light). 


Fig.  151.   (x  1?5  diameters)  Hedruth,  Cornwall,  Similar 
to  Fig.  150.  The  relations  elsewhere  and  the 
structure  of  the  chrdcocite  indicates  that  the 
chaloocite  is  R  replacement  of  bornite.  The 
preservation  of  these  veinlika  strips  may 
possibly  be  due  to  outgoing  iron.  Chalcopyrite 
frequently  occurs  along  their  centre  lines. 


Bornite 

Fist.  152.   (x  150  diameters}   COFPer  Queen.  Bisbee,  Ariz.- 
(dark),  partially  altered  to  ohalcocite 
(lisht),  which  develops  the  lattice  struc- 
ture in  the  bornite. 


Fig.  153.   (x  50  diameters)  Calument  *nd  Arizona  Mine, 
Biabee,  Arizona.  Chaloopyrite  grains  with 
halos  of  bornite  in  n  field  of  chalcocite. 
Cores  of  pyrite  in  a  few  cases. 


Fig.  151J-.   (x  115  diameters)  Bonanza  Mine,  Kannecott, 
Alaska.  Bornite  (dark)  partially  replaced 
by  intricate  chaloopyrite  lattice  (light). 
Vein-like  strips  of  bornite  remain,  which 
at  this  magnification  make  it  seem  certain 
that  the  sequence  j  j  the  reverse  of  that 
stated.  For  discussion,  see  page  359. 


Fie.  1~5.   (x  itO  diameters)  Butte,  Montana.  Bornite 
(light  gray)  partially  altered  to  covellite 
(dark)  alon?;  veinlets,  with  adjacent  chalco- 

rite  (white)  ^nd  pyrite  (rough,  with  high 
relief)  untouched. 


435. 


PLATE  XXXVII 
'   ^Mqihed   Surfaces   of  Ores 


f  ror.   3haata   Co  .",   Co  lif  orni  g  ._ 
(Photographed  by  J.  Murdoch) 


1^6.      (x         diameters)     From  Sutro  Mine.     Bornite 
(dark)   and   chalcooite    (lip;ht).     The   chalco- 
cita  possesses  s  grained  structure  and  a 
mottled  appearance  shown  by  the  variation  in 
color  on  the  polished  surface.     Not«  minute 
sub0;raphic  forma  of  the  bornite  in  the  chaloo- 
cite. 


Fig.   157-      (*       diameters)     From  Sutro  Mine.     The  aame 
field  as  in  figure  156,  but  etched  with  ni- 
tric Bold,  revealing  the  orthorhombic  nature 
of  the  chnlcooits,   and  showing  the  different 
orientations  of  ths  individual  chalcooite 
grains.     The  fine   linaa  of  the  etch-pattern 
of  the  bornite  are  shown  faintly,   especially 
on  the  right   side  of  the  ch^lcocite  field, 
and  the  structure  is  emphasized  by  the  char- 
acteristic blocks  which  have  sprung  out  be- 
tween the  cracks. 


Fig.    156".      (x       diameters)     From  Sutro  Mine.     Bornite 

(dark)   and  mottled  ohalcocite    (light)   in  re- 
lations similar  to  those  shown  in  figures 
157. 


1*^9.      (x       diameters)     From  Afterthought  Mine.      Cal- 
oite   (black),  partially  replaced  along  ita 
cleavages  by  bornite    (grwy)   «nd  ohalcocite   (light). 

n.   chplcocite  i  a  believed  to  bs  a  replacement 
of  bornite. 


436 


Plate  XXXVIII. 

Photographs  of  Polished  Surfaces  of  Ore. 
(Photographed  by  Murdoch). 

Fig.  160.   (x   diarr.eters)  Butte,  Montana.   Chalcopyrite 
(white)  developed  in  bornite  (darkj  in  the  lat- 
tice structure. 


Fig.  161.   (x   diameters)  Butte,  Montana.   Chalcopyrite 
(white)  developed  in  bornite  (dark)  in  part  in 
the  lattice  structure. 


Fig.  162.  (x  45  diameters)  Artificial  products  close 
to  ch^lcopyrite  an  1  bornite  in  composition, 
crystallized  from  a  melt  in  the  Geophysical 
Laboratory.  (chalcopyrite,  light;  bornite, 
dark). 


Fig.  163.   (x  85  diameters)  Redruth,  Cornwall.  Bor- 
nite crystal  containing  symmetrically  dis- 
tributed chalcopyrite  in  the  lattice  struc- 
ture* 


Fig.  164.   (x  770)  Seven  Devils,  Idaho.  Exceedingly 
fine  lattice  of  chalcopyrite  in  bornite. 


Fig.  165.   (x  770)  Seven  Devils,  Idaho.  Exceedingly 
fine  lattice  of  chaloopyrite  in  bornite. 


1940 


GENERAL  LIBRARY  -  U.C.  BERKELEY 


U.C.BERKELEY 
ENGINEERING  LIBRARY 


